Full text of "A treatise on rocks, rock-weathering and soils"

Branch of the College of Agriculture
Davis, California

ROCKS, ROCK-WEATHERING AND SOILS

A TREATISE ON

ROCKS, ROCK-WEATHERING
AND SOILS

GEORGE P. MERRILL

HEAD CURATOR OF GEOLOGY IN THE UNITED STATES NATIONAL MUSEUM, AND PROFESSOR OF

GEOLOGY IN THE GEORGE WASHINGTON UNIVERSITY, WASHINGTON, D. C.J AUTHOR OF

"STONES FOR BUILDING AND DECORATION" ; "THE NON-METALLIC MINERALS" ;

" CONTRIBUTIONS TO A HISTORY OF AMERICAN GEOLOGY " J ETC.

Neto $orfe
THE MACMILLAN COMPANY

LONDON: MACMILLAN & CO., LTD.
1906

Set up and electrotyped. Published March, 1897. Reprinted November, 1904.
New Edition Revised throughout, December, 1906.

PRESS OF

THE NEW ERA PRINTING COMPANY
LANCASTER, PA.

' * THE ruins of an older world are visible in the present structure
of our planet; and the strata which now compose our continents
have been once beneath the sea, and were formed out of the waste
of pre-existing continents. The same forces are still destroying,
by chemical decomposition or mechanical violence, even the hardest
rocks, and transporting the materials to the sea, where they are
spread out, and form strata analogous to those of more ancient
date. ' ' HUTTON.

185882

PREFATORY NOTE

IN the work here presented the writer has endeavored to
bring together in systematic form the results of several years'
study of the phenomena attendant upon rock degeneration
and soil formation. Although beginning with a discussion
of rocks and rock-forming minerals, the work must be con-
sidered in no sense a petrology as this word is commonly
used. What is here given relative to the origin, structure,
and composition of rock masses is regarded as an essential
introduction to the chapters on rock-weathering. The por-
tion dealing with the structure and composition of the resultant
materials is an essential corollary to these same chapters.

It is believed that no apology is necessary for bringing out the
present work. The origin, structure, and mineral composition of
rocks, particularly the eruptive varieties, are matters which have
of late received much attention. In fact, it is to these rocks that
the petrol ogists have devoted their best efforts. Since the intro-
duction of the microscope into petrographic work, there has, how-
ever, been very little time devoted to the study of rocks in a
weathered condition. The chemists have made analyses, but have
disregarded the physical and mineralogical nature of the material
analyzed. Other workers have studied the physical properties
of rocks decayed, in the form of soils, but have in their
turn disregarded their mineral and chemical nature. The
writer has aimed to bring together here such results obtained
by these workers in divers fields as it is believed will be for
the mutual benefit of all concerned. The state of comminu-
tion reached by rocks during the processes of long-continued,
secular decay, and the amount of leaching such have under-
gone, are certainly of as much practical interest to the agri-
culturist as of theoretical interest to the geologist.

vii

vili PEEFATOEY NOTE

To the one, these residues are essential to the life and well-
being of man through furnishing the soils from whence is
derived directly and indirectly the food for life's sustenance;
to the other they are but transitory phases in the earth's his-
tory, representing the materials from which, through a process
of fractional separation by running waters, have been made
up the thousands of feet of secondary rocks which to-day
occupy so large a portion of its surface.

The very general scheme of classification adopted in the
treatment of the unconsolidated clastic materials may at first
seem disappointing. It was, however, the writer's aim to in-
troduce into the preliminary volume as few new terms as pos-
sible, to use only those which through years of service have
become a part of the language. It is of course possible that in
his desire to avoid any possible confusion such as might arise
through putting forward a purely tentative classification he was
overcautious.

It is possible, further, that in numerous instances it may
appear that too much reliance was placed upon single analyses,
particularly in the discussions relating to the character of
decomposed material. Regarding this it can only be said that
in those instances upon which most reliance was placed, the
materials were not merely collected by the author himself, but
that he made his own chemical analyses and microscopic deter-
minations as well. It is believed that the fresh and residual
materials examined were in each instance as truly representative
of the same rock mass, as would be samples of fresh rock col-
lected equal distances apart. In all cases special effort was
made to obtain material concerning the lithological identity
of which there could be no doubt, and in the majority of cases
the residuary matter was collected from positions immediately
overlying the still unaltered rock. Where such a procedure
was impossible, especial care was exercised to obtain only such
as was originally of the same lithological nature as the fresh
rock, and which had suffered no contamination from extrane-
ous sources. The fact that stratified rocks are likely to vary

PEEFATOEY NOTE IX

so greatly within short distances, and hence that a residual clay
cannot be relied upon to represent the residue from rocks of
the same nature immediately underlying, will serve to explain
in part the author's limiting himself so largely to a discussion
of massive eruptive materials. It is pleasing to note that later
analyses, by other and perhaps better workers, have fully cor-
roborated the results first obtained.

In the preparation of the revised edition many errors have
been corrected, matter that proved non-essential eliminated,
and a considerable number of new analyses and illustrations
introduced.

As will be readily perceived by those at all acquainted with
the general literature, the publications of the U. S. Geological
Survey, the U. S. National Museum, and the Bulletins of the
Geological Society of America have been drawn upon to furnish
materials for illustration. The writer, as before, is under special
obligation to Dr. Milton Whitney of the U. S. Department of
Agriculture for many of the mechanical analyses given, and to
Professor L. H. Merrill of the Maine Experiment Station for
numerous criticisms and suggestions.

GEOEGE P. MEEEILL.

U. S. NATIONAL MUSEUM, January, 1906.

CONTENTS

PART I

THE CONSTITUENTS, PHYSICAL AND CHEMICAL

PROPERTIES, AND MODE OF OCCURRENCE

OF ROCKS

PAGE

I. INTRODUCTORY: ROCKS DEFINED 1

II. THE CHEMICAL ELEMENTS CONSTITUTING ROCKS ... 4

III. THE MINERALS CONSTITUTING ROCKS 9

IV. THE PHYSICAL AND CHEMICAL PROPERTIES OF ROCKS . . 30

1. The Structure of Rocks, macroscopic and microscopic 30

2. The Specific Gravity of Rocks 40

3. The Chemical Composition of Rocks .... 41

4. The Color of Rocks 42

V. THE MODE OF OCCURRENCE OF ROCKS 45

PART II

THE KINDS OF ROCKS

GENERALITIES, AND CLASSIFICATION 52

I. IGNEOUS ROCKS : ORIGIN OF, AND CLASSIFICATION ; RELATION-
SHIP EXISTING BETWEEN PLUTONIC AND EFFUSIVE ROCKS 55

1. The Granite-Liparite Group 61

2. The Syenite-Trachyte Group 68

3. The The Foyaite-Phonolite Group .... 73

4. The Diorite-Andesite Group 76

5. The Gabbro-Basalt Group 80

6. The Theralite-Basanite Group 88

xii CONTENTS

PAGE

7. The Peridotite-Limburgite Group 89

8. The Pyroxenite-Augitite Group 93

9. The Leucite-Nepheline Rocks 96

II. AQUEOUS ROCKS 99

1. Rocks formed through Chemical Agencies ... 99

(1) Oxides . 100

(2) Carbonates 104

(3) Silicates . . 106

(4) Sulphates 109

(5) Phosphates Ill

(6) Chlorides Ill

2. Rocks formed as Sedimentary Deposits .... 112

(1) Rocks composed mainly of Inorganic Material . 113

(1) The Arenaceous Group: Psammites . 113

(2) The Argillaceous Group: Pelites . 117

(3) The Calcareous Group: Calcareous Con-
glomerate and Breccia .... 121

(4) The Volcanic Group: Tuffs . . .122

(2) Rocks composed mainly of debris from Plant

and Animal Life 123

(1) The Siliceous Group: Diatom Earth . 123

(2) The Calcareous Group : Limestone, Marl,

etc .124

(3) The Carbonaceous Group: Peat, Lignite,

and Coal 129

(4) The Phosphatic Group . . . .131

III. JEOLIAN ROCKS .... .133

Volcanic Dust; Dune Sands, etc. . . . . 133

IV. METAMORPHIC ROCKS ... .... 135

Agencies and Results of Metamorphism and Metasomatosis 135

1. Stratified or Bedded .... .141

(1) The Crystalline Limestones and Dolomites 141

2. Foliated or Schistose 142

(1) The Gneisses 142

(2) The Crystalline Schists . . . .146

CONTENTS xiii

PART III
THE WEATHERING OF ROCKS

PAGE

I. STATEMENT OF GENERAL PROBLEM; PRINCIPLES INVOLVED IN

ROCK-WEATHERING 150

Weathering defined 151

1. Action of the Atmosphere 154

(1) Nitrogen, Nitric Acid, and Ammonia of

the Atmosphere 154

(2) Carbonic Acid of the Atmosphere . .156

(3) Oxygen of the Atmosphere . . . 158

(4) Effects of Heat and Cold . . .158

(5) Effects of Wind 163

2. Chemical Action of Water 165

(1) Oxidation ... ... 165

(2) Deoxidation 166

(3) Hydration 166

(4) Solution 168

3. Mechanical Action of Water and of Ice . . . 175

4. Action of Plants and Animals 180

II. CONSIDERATION OF SPECIAL CASES 185

-*- (1) Weathering of Granite 185

(2) Weathering of Nepheline (Elaeolite) Syenite . . 196

(3) Weathering of Phonolite 197

(4) Weathering of Diabase 198

(5) Weathering of Basalt 205

(6) Weathering of Diorite 207

(7) Weathering of Andesite 207

(8) Weathering of Ultra Basic Rocks . . . .208

(9) Weathering of Sedimentary Rocks . . . .212
(10) Resume: Importance of Hydration; Loss of Constitu-
ents ; Relative Durability of Various Minerals ; Dis-
cussion of Processes involved in Feldspathic De-
composition 220

xiv CONTENTS

PAGE

III. THE PHYSICAL MANIFESTATIONS OF WEATHERING . . . 227

(1) Disintegration without Decomposition . . . 227

(2) Weathering influenced by Crystalline Structure . . 229

(3) Weathering influenced by Structure of Rock Masses . 230

(4) Weathering influenced by Mineral Composition . . 234

(5) Results due to Position 238

(6) Induration on Exposure 240

(7) Changes in Color incidental to Weathering . . 243

(8) Relative Amount of Material removed in Solution , 245

(9) Incidental Surface Contours 246

(10) Effacement of Original Characteristics . . . 249

(11) Simplification of Chemical Compounds incidental to

Weathering 252

(12) Other Results incidental to Decomposition and

Erosion 253

IV. TIME CONSIDERATIONS . . . 255

(1) Rate of Weathering influenced by Texture . . . 255

(2) Rate of Weathering influenced by Composition . . 256

(3) Rate of Weathering influenced by Humidity . . 257

(4) Rate of Weathering influenced by Position . . . 257

(5) Relative Rapidity of Weathering among Eruptive and

Sedimentary Rocks . . 258

(6) Time Limit of Decay .... .260
_ (7) Relative Rapidity of Weathering in Warm and Cold

Climates .263

(8) Difference in Kind of Weathering in Cold and Warm

Climates .- 269

- (9) Extent of Weathering .... .271
(10) Relative Amounts of Materials lost through Weather-
ing in Hilly and Plains Regions .... 273

PART IV
TRANSPOETATION AND REDEPOSITION OF ROCK DEBRIS

1. ACTION OF GRAVITY 274

2. ACTION OF WATER AND ICE 275

3. ACTION OF WIND 280

CONTENTS XV

PART V
THE REGOLITH

PAGE

I. CLASSIFICATION AND GENERAL DESCRIPTION .... 287

1. Sedentary Materials 288

(1) Residuary Deposits: Residual Sands and Clays;

Terra Rossa; Laterite, etc 289

(2) Cumulose Deposits: Peat; Muck and Swamp

Soils in part 301

2. Transported Materials 307

(1) Colluvial Deposits: Talus, Cliff Debris and Ma-

terial of Avalanches 307

(2) Alluvial Deposits: Modern Alluvium; Delta;

Sea-coast Swamps; Loess; Adobe in part;
Champlain Clays; Beach Sands and Gravel . 308

(3) Eolian Deposits: Wind-blown Sand; Sand

Dunes; Volcanic Dust 331

(4) Glacial Deposits: Moraine Material; Eskers;

Drumlins, etc 338

3. The Soil 345

, (1) The Chemical Nature of Soils . . . .345

(2) The Mineral Composition of Soils . . .362

(3) The Physical Condition of Soils . . . .367

(4) The Weight of Soils 371

(5) The Kinds and Classification of Soils . . .371

(6) The Color of Soils 373

(7) The Age of Soils 375

(8) Soils as Affected by Plant and Animal Life . 378

ILLUSTRATIONS

FULL-PAGE PLATES

PLATE 1 Frontispiece

Stone Mountain, Georgia. A Residual Boss of Granite.
From a photograph by J. K. Hillers.

FACING PAGE

PLATE 2 30

Porphyritic and Flow Structures.
PLATE 3 32

Slaggy and Vesicular Structures.
PLATE 4 34

Brecciated Structures.
PLATE 5 38

Microscopic Structures of Rocks.
PLATE 6 . . .61

FIG. 1, Liparite, Nevadite form.

FIG. 2, Liparite, Rhyolite form.

FIG. 3, Liparite, Obsidian form.

FIG. 4, Liparite, Pumiceous form.
PLATE 7 .76

FIG. 1, Orbicular Diorite.

FIG. 2, Granite Spheroid.
PLATE 8 100

FIG. 1, Botryoidal Hematite.

FIG. 2, Septarian Nodule.
PLATE 9 106

View in Limestone Cavern.
PLATE 10 113

FIG. 1, Shell Limestone.

FIG. 2, Shell Limestone (Coquina).

FIG. 3, Crinoidal Limestone.

xvii

iviii ILLUSTEATIONS

FACING PAGE

PLATE 11 125

FIG. 1, Pisolitie Limestone.
FIG. 2, Oolitic Limestone.

PLATE 12 135

Banded and Foliated Gneisses.

PLATE 13 150

FIG. 1, Glaciated and Exfoliated Granite, Cathedral Lake, in
the Sierras. From a photograph by G. K. Gilbert,
U. S. Geological Survey.

FIG. 2, Weathered Biotite Granite near Morrison Creek, Yo-
semite, California. From photograph by H. W.
Turner, U. S. Geological Survey.

PLATE 14 168

FIG. 1, Honeycomb Weathering in Limestone, near Living-
stone, Montana. From a photograph by C. D.
Walcott, U. S. Geological Survey.
FIG. 2, Corroded Surface of Pyroxenic Limestone.
FIG. 3, Corroded Surface of Homogeneous Limestone.

PLATE 15 175

FIG. 1, Diorite Boulder Split by Frost.
FIG. 2, Exfoliated Granite Boulder.

PLATE 16 185

Weathered Granite, District of Columbia. From a photo-
graph by G. P. Merrill.

PLATE 17 199

Weathered Diabase, Medford, Mass. From a photograph by
G. H. Barton.

PLATE 18 227

FIG. 1, Exfoliated Granite in the California Sierras. From a
photograph by H. W. Turner, U. S. Geological
Survey.

FIG. 2, Rock Basin, one meter in diameter, in Granite. From
a photograph by H. W. Turner, U. S. Geological
Survey.

FIG. 3, Disintegrating Granite, Ute Pass, Colorado. From
a photograph by W. H. Jackson.

ILLUSTRATIONS xix

FACING PAGE

PLATE 19 230

Weathering Controlled by Jointing, Summit of Mt. Ktaadn,
Maine. From a photograph by L. H. Merrill.

PLATE 20 234

FIG. 1, Weathered Mica Schist, coast of Cape Elizabeth,

Maine.

FIG. 2, Sandstone Bored by Bees.
FIG. 3, Glaciated Limestone.

PLATE 21 239

Weathering of Horizontally Bedded Jurassic Sandstone and
underlying thin bedded Calcareous Rocks, near Bluff, Utah.
From a photograph by Whitman Cross, U. S. Geological
Survey.

PLATE 22 244

FIG. 1, Weathered Boulder of Oriskany Sandstone.

FIG. 2, Concentric Weathering in Diabase.

FIG. 3, Zonal Weathering in Argillite.

FIG. 4, Sandstone Showing Induration Along Joint Planes.

PLATE 23 247

FIG. 1, Sinkhole near Knoxville, Tenn.
FIG. 2, Marble Beds Corroded by Water.

PLATE 24 . ' 255

Residuary Quartzite, Bakalsk Iron Mines, Russian Urals.

PLATE 25 274

Rock Disintegration and the Formation of Talus, Mount
Sneffels, Colo. From a photograph by Whitman Cross,
U. S. Geological Survey.

PLATE 26 280

FIG. 1, Forest Destroyed by Wind-blown Sand. From a
photograph by I. C. Russell, U. S. Geological
Survey.

FIG. 2, Wind Drifts and Wind Erosion, White Valley, West-
ern Utah. From a photograph by G. K. Gilbert,
U. S. Geological Survey.

PLATE 27 287

Landslide at Rico, Colorado. From a photograph by Whit-
man Cross, U. S. Geological Survey.

xx ILLUSTEATIONS

FACING PAGE

PLATE 28 322

FIG. 1, Leda Clays at Lewiston, Maine. From a photograph

by L. H. Merrill.

FIG. 2, Beds of Volcanic Dust in Gallatin Co., Montana.
From a photograph by G. P. Merrill.

PLATE 29 338

FIG. 1, Section of Glacial Till. From a photograph by G. F.

Wright.
FIG. 2, Drift Bowlders in Walls of Glaciated Region.

PLATE 30 344

Kames near Whitewater, Wisconsin. From Professional
Paper No 34, U. S. Geological Survey.

PLATE 31 378

Gullied Field near Marion, North Carolina. From Profes-
sional Paper No. 37, U. S. Geological Survey.

FIGURES IN TEXT

FI. PAGE

1. Augite partially altered into Hornblende .... 36

2. Mounted Thin Section of Rock . 40

3. Microscopic Structure of Muscovite-Biotite Granite, Hallo-

well, Maine . . 63

4. Microscopic Structure of Diabase, Weehawken, New Jersey 83

5. Microscopic Structure of Peridotite (Porphyritic Lherzolite) 91

6. Microscopic Structure of Pyroxenite 94

7. Microscopic Structure of Oolitic Limestone .... 105

8. Pyroxene partially altered into Serpentine .... 108

9. Microstructure of Sandstone . 114

10. Section through Lake Basin, showing Bed of Diatom Earth 124

11. Microstructure of Oolitic Limestone 126

12. Microstructure of Fossiliferous Limestone .... 127

13. Microstructure of Quartzite 137

14. Microstructure of Crystalline Limestone .... 142

15. Microstructure of Gneiss 143

16. Rock Undermined by Windblown Sand . . . .164

17. Influence of Joints in the Production of Boulders 230

ILLUSTEATIONS xxi

F1G - PAGE

18. Exfoliation of Granite, Stone Mountain, Georgia . . . 231

19. Concentric Exfoliation of Granite, Canada .... 232

20. Microstructure of Sandstone, with Large Absorptive Power 256

21. Microstructure of Diabase, with relatively Little Absorptive

Power 256

22. Flint Implement showing Weathered Surface . . . 261

23. Sketch showing Pre-Pala3ozoic Decay of Rocks . . . 263

24. Diagram showing Direction and Rate of Motion of Soil . 275

25. Diagram showing Flood Plain of River .... 277

26. Angular Outlines of Particles in Residual Soil from Gneiss . 289

27. Section across Central Kentucky, illustrating Inherited Char-

acteristics of Soils ...... 291

28. Angular Quartz Particles from Decomposed Gneiss . . 292

29. Outlines of Kaolinite Crystals and Kaolin Particles . . 297

30. Section across Small Lake 302

31. Talus Slopes 308

32. Alluvial Plains .311

33. Outlines of Particles in Chinese Loess 317

34. Particles washed from Leda Clays 323

35. Cross-section of Marine Marsh 326

36. Quartz Granules in Beach Sand 331

37. Outlines of Particles of Glass in Volcanic Dust . . .337

38. Section through Carboniferous Soil 376

39. Section showing Varying Character of Residual Soil . . 377

40. Section through Ant Nest 379

41 and 42. Sections showing the Effect of Tree Roots in Soil . 384

Fig. 1, after G. W. Hawes; 5 and 6, after G. H. Williams; 16, after
G. K. Gilbert; 18 and 22, after Robert Bell; 10, 23, 24, 26, 29, 30, 31, 34,
37, 38, 39, 40, and 41, after Shaler, Twelfth Annual Report United
States Geological Survey, 1890-1891.

- 1 V E R S
V

ITY\

ROCKS, ROCK-WEATHERING,
AND SOILS

PART I

THE CONSTITUENTS, PHYSICAL AND CHEMICAL
PROPERTIES, AND MODE OP OCCURRENCE OP
ROCKS

I. INTRODUCTORY

A ROCK is a mineral aggregate; more than this, it is an
essential portion of the earth's crust, a geological body occu-
pying a more or less well-defined position in the structure of
the earth, either in the form of stratified beds, eruptive masses,
sheets or dikes, or in that of veins and other chemical deposits
of comparatively little importance as regards size and extent.
In giving this definition, origin, chemical composition, and state
of aggregation of the individual particles are for the time
ignored. From a strictly geological standpoint, the beds of
loose sand, and even the water of the ocean itself, may be
considered as rocks, and either, under favorable circumstances,
may undergo a process of induration such as shall be produc-
tive of the condition of solidity commonly ascribed to rocks
by the popular mind.

In ever-varying conditions as regards compactness, color,
texture, and structure, rocks form the entire mass of the globe
so far as it is as yet made known to us, with the exception of a
scarcely appreciable proportion of organic matter. It is rock
which forms the substance of mountain ranges and the vast
stretches of valley and plain. It is from the rocks that we
gain our food, our fuel, and the supplies of metal which are
seemingly so essential to our Well-being; we cannot ignore
2 l

2 INTRODUCTORY

them, even if we would. We borrow from the rocks that which
is essential to our life to-day, but when that brief day is ended
return it once more, with neither loss nor gain, to its original
source.

Those portions of the earth's crust which are available for
study comprise at best but a few thousand vertical feet, though
from the fact that the stratified rocks have been extensively
thrown out of their original, horizontal position, and again
eroded, we are enabled to measure their thickness, and may
claim to know with a reasonable degree of accuracy the char-
acter of the material forming this crust down to a depth of
perhaps twenty miles. 1 Throughout all this vast thickness,
comprising millions upon millions of cubic feet, in weight far
beyond all comprehension, is found a constant recurrence of
materials alike in composition and similar in origin to those
upon the immediate surface. There is at times, as noted later,
a difference in structure, due to metamorphism, between the
older, deeper lying portions and those of more recent origin, but
the ultimate composition is essentially the same, and all the
knowledge thus far gained points to a wonderful unity in na-
ture's methods, and shows with seeming conclusiveness that the
geological agencies of the past, the methods by which rocks were
made and again destroyed, differed in no essential particular
from those in progress to-day. What these processes were,
how they operated, and with what results, it shall be our aim
here to set forth.

Among the many interesting, and at first thought seemingly
unaccountable, things encountered in the progress of our work,
not the least is the fact that so large a proportion of natural
objects are more or less out of harmony with their surroundings.
Throughout life every organic being is in a constant struggle
with the elements to preserve that life, fulfil all its functions,
and gratify its natural desires. No sooner does life depart than
decomposition and disintegration ensue. As with organic beings,
so with inorganic substances. Every mass of rock pushed up
by the faulting and folding of the earth's crust, exposed by
denudation, or erupted as molten matter from the earth's in-
terior, finds almost at once that its various elements, in their
existing combinations, are not in harmony with their environ-

1 The total mean depth of the f ossilif erous formations of Europe as stated
by Geikie (Text-book of Geology, p. 675) has been set down as 75,000 feet.

INTRODUCTOEY 3

ment. The summer's heat and winter's cold, the chemical action
of atmospheres and acidulated rains, combine their forces; a
breaking up ensues, to be succeeded by new combinations and
perhaps reconsolidations more in keeping with existing circum-
stances. An intermediate product in all this endless cycle
of change, of disintegration and recombination, is a compara-
tively thin, superficial mantle of loose debris, which, mixed with
more or less organic matter, nearly everywhere covers the land,
and by its combined chemical and mechanical properties fur-
nishes food and foothold for myriads of plants, and hence,
indirectly, sustenance for man and beast. In brief, what is
commonly known as soil is but disintegrated and more or less
decomposed rock material, intermingled, perhaps, with organic
matter from plant decay. Such being the case, a study of the
processes of rock weathering and the transportation, deposition,
and physical properties of the resultant debris, is but a study
of the origin of soils on the broadest and most comprehensive
basis. Their study belongs as legitimately to the realm of
geology as does that of any subject relating to rock formation
or other phases of the earth's history.

Accepting the above, the various phases of the subject will be
taken up in the following order: (1) the elements which in their
single or combined state make up the minerals; (2) the minerals
which make up the rocks; (3) the rocks themselves, with par-
ticular reference to their mineralogical and chemical natures;
(4) the breaking down or degeneration of rocks through proc-
esses in part chemical and in part mechanical; and (5) the
result of this clasmatic process as manifested in the production
of clay, sand, gravel, and incidental soil. There are other points
which will be touched upon more briefly, in order to make the
work systematic, as the action of wind and water in assorting
and redepositing rock debris and tending to reduce the land
surface to one general level.

II. THE CHEMICAL ELEMENTS CONSTITUTING

ROCKS

Although there are upwards of seventy elements now known,
but sixteen occur in any abundance or form more than an ex-
tremely small proportion of the material of the earth 's crust. In-
deed, of this number probably fully one-half, taken collectively,
will not constitute more than 4 or 5% of the earth's crust so far
as know T n. These sixteen, arranged according to their chemical
properties and the order of their abundance, are as follows:
oxygen, silicon, carbon, sulphur, hydrogen, chlorine, phosphorus,
'fluorine, aluminum, calcium, magnesium, potassium, sodium,
iron, manganese, and barium. The eight more important, with
their approximate percentage amounts as shown by the most
recent calculation, 1 are as follows :

Oxygen 47.02 Calcium 3.50

Silicon 28.06 Magnesium .... 2.62

Aluminum .... 8.16 Sodium . . . . . 2.63

Iron 4.64 Potassium .... 2.32

It must not be imagined, however, that these elements exist
for the most part in a free or uncombined state: on the con-
trary, in the majority of cases so great is their affinity for one
another that it is only momentarily, or under abnormal con-
ditions, that they are met with at all in this elementary form.
The elements which are most common in the free state, though
even these occur more commonly combined with others, are,
(1) the gas oxygen, and (2) the solids, carbon, sulphur, and,
more rarely, iron. Still more rarely, and under such abnormal
conditions, as exist during volcanic eruptions, are found the
free gases, hydrogen, chlorine, and, according to some authori-
ties, fluorine. The gas nitrogen, although so abundant a con-
stituent of the atmosphere, is, as a primary constituent of the

1 By R W. Clarke, Bull. 168, U. S. Geol. Survey, 1900. The figures given
in the first edition of the present work as quoted from Eoscoe and Schor-
lemmer's Treatise on Chemistry, were: Oxygen 44.0-48.7, Silicon 22.8-36.2,
Aluminum 9.9-6.1, Iron 9.9-2.4, Calcium 6.6-0.9, Magnesium 2.7-0.1, Sodium
2.4-2.5, Potassium 1.7-0.1.

OXYGEN 5

earth's crust, almost wholly unknown, and needs no considera-
tion at this stage of our work.

Oxygen, as is well known, is the active, even the aggressive,
principle of the atmosphere, of which it constitutes about one-
fifth by bulk. Combined with other elements, it is of great
geological importance, being estimated, as noted above, to con-
stitute 47.02% of the entire mass of the earth's crust; that is
to say, could this crust be resolved into its original elements,
the oxygen thus liberated would be found very nearly equal to
all the other elements taken together. The simpler forms of
oxygen compounds are known as oxides, and of these the oxide
of hydrogen, water (H 2 0), is by far the most common, and,
anomalous as it may at first seem, is a true mineral. Aside from
being so essential to human life, oxygen is a very potent factor
in the manifold changes which are constantly taking place in
the more superficial portions of the earth's crust.

Silicon. Next to oxygen silicon is the most abundant of
the earth's constituents, though it exists only in combination,
either as an oxide (silica), or with other elements to form
silicates. In these two forms it is the predominating con-
stituent in all but the calcareous rocks. As silica (SiO 2 ), or
quartz, it forms one of the most indestructible of natural com-
pounds, and hence is to be found as the prevailing constituent
in nearly all sands and soils.

Aluminum is next to oxygen and silicon probably the most
important element when regarded from the present standpoint.
It occurs mainly in combination with silicon and oxygen, form-
ing an important series of minerals known as aluminous sili-
cates. As a sesquioxide it is well known in the minerals
corundum and beauxite.

Iron, although less abundant than either oxygen or silicon,
occupies a very important place as a rock constituent, owing to
the variety of compounds of which it forms a part, as well as
to the decided colors which are characteristic of its oxides and
of the iron-bearing silicates. The. most conspicuous forms of
iron on the immediate surface of the earth are the oxides, but
which at greater depths, or where the atmosphere has as yet
exercised less influence, give way to carbonates, sulphides, and
silicates.

Iron, although so common in combination with other elements,
occurs but rarely free, owing to its affinity for oxygen.

6 CHEMICAL ELEMENTS CONSTITUTING THE KOCKS

Calcium is a very important element of the earth's crust,
although it has been estimated to compose only about 3.5% of
its mass. Its most conspicuous form of occurrence is in com-
bination with carbon dioxide, forming the mineral calcite
(CaC0 3 ), or the rock limestone. In this form it is slightly
soluble in water containing carbonic acid, and hence has be-
come an almost universal ingredient of all natural waters,
whence it furnishes the lime necessary for the formation of
shells and skeletons of the various tribes of mollusca and corals.
In combination with sulphuric acid, calcium forms the rock
gypsum. It is also an important constituent of many silicates.

Magnesium is found in combination with carbonic acid as
carbonate, forming thus an essential part of the mineral mag-
nesite and the rock dolomite. The bitter taste of sea-water and
some mineral waters is due to the presence of salts of magnesia.
In combination with silica it forms an essential part of such
rocks as serpentine, soapstone, and talc.

Potassium combined with silica is an important element in
many mineral silicates, as orthoclase, leucite, and nepheline.
In smaller amount it is found in silicates of the mica, amphi-
bole, and pyroxene groups. The following table will serve to
show the varying amounts of potash (K 2 0) in rocks of various
kinds :

Granite 2.6 to 6.50%

Diorite 0.1 to 2.42%

Basalt 0.058 to 0.50%

Gabbro 0.00 to 0.93%

Limestone 0.19 to 1.22%

Sandstone 0.00 to 3.30%

Slate (fissile argillite) 0.00 to 3.83%

As a chloride, potassium is invariably present in sea-water,
and as a nitrate it forms the mineral nitre, or saltpetre.

Sodium. The most common and wide-spread form of the
element sodium is the compound with chlorine known as sodium
chloride (NaCl) or common salt. In this form it is the most
abundant of the salts occurring in sea-water, and constitutes
also rock masses of no inconsiderable dimensions interstratified
with other rocks of the earth's crust. Combined with silica,
lime, and alumina, sodium is an important constituent of the
soda-lime feldspars, and of numerous other silicate minerals.

SODIUM 7

In the form of carbonate and sulphate it occurs as an incrusta-
tion on the surface of the ground, or disseminated throughout
the soils in poorly drained portions of arid countries, giving
rise to the so-called "alkali soils," for which such regions are
frequently noted. As a nitrate, sodium occurs in the desert
regions of Chili, forming the soda nitre so valuable for fer-
tilizing purposes.

Manganese is, next to iron, the most abundant of the heavy
metals. It occurs as an oxide, carbonate, or in combination with
two or more other elements as a silicate.

Barium is found mainly combined with sulphuric acid, to
form the mineral barite or heavy spare. It sometimes occurs
as a carbonate, and more rarely as a silicate.

Phosphorus, although existing in comparatively insignificant
proportions, is nevertheless an important element. In nature
it occurs only in combination with various bases, principally
lime, to form phosphates. In this form it is found in the bones
of animals, the seeds of plants, and constitutes the essential
portions of the minerals apatite and phosphorite. Though
small in proportion, phosphorus is a very important constituent
of any fertile soils. Its chief source, in the older, crystalline
rocks, is the mineral apatite. Where found in the secondary
rocks, as limestones and marls, it is evidently derived from
animal remains. (See p. 131.) Analyses have shown that the
amount of phosphorus in rocks rarely exceeds 1% (calculated
as P 2 5 ), and usually falls much lower, being most abundant
in the basic eruptives. The following table will serve to show the
small percentages of this constituent in rocks of various kinds:

Granite 0.07 to 0.25%

Diorite 0.18 to 1.06%

Basalt 0.03 to 1.18%

Limestone 0.06 to 10.00 %

Shale 0.02 to 0.25%

Sandstone 0.00 to 0.1 %

Carbon. Of the solid elements occurring free, or uncom-
bined, carbon is by far the more abundant, being found in the
forms known as diamond and graphite, or when quite impure
as coal. In combination as a dioxide (CO,), it forms the well-
known carbonic acid gas, which, like oxygen, is a powerful
agent in bringing about important changes in the rocks with

8 CHEMICAL ELEMENTS CONSTITUTING THE EOCKS

which it comes in contact. Free sulphur occurs more^rarely,
being as a rule a product of volcanic activity, or due to the
reduction of the sulphides and sulphates of the metal with
which it more commonly exists in combination.

III. THE MINERALS CONSTITUTING ROCKS

A rock, as previously stated, is a mineral aggregate. As a
rule, the number of mineral species constituting any essential
portion of a rock is small, seldom exceeding three or four. In
common crystalline limestones, the only essential constituent
is the mineral calcite; granite, on the other hand, is, as a rule,
composed of minerals of three or four independent species.
The mineral composition of rocks in general is greatly simpli-
fied by the wide range of conditions, under which their chief
constituents can be formed, thus allowing their presence in
rocks of all classes and of whatever origin. Quartz, feldspar,
mica, the minerals of the hornblende or pyroxene group, can be
formed in a mass cooling from a state of fusion; they may be
crystallized from solution, or be formed from volatilized prod-
ucts. They are therefore the commonest of minerals and rarely
excluded from rocks of any class, since there is no process of
rock formation which determines their absence. Moreover, most
of the common minerals, like the feldspars, micas, hornblendes,
pyroxenes, and the alkaline carbonates, possess the capacity of
adapting themselves to a very considerable range of composi-
tions. In the feldspars, for example, lime, soda, or potash may
replace each other almost indefinitely, and it is now commonly
assumed that true species do not exist, all being but isomorphous
admixtures passing into one another by all gradations, and the
names albite, oligoclase, anorthite, etc., are to be used only as
indicating convenient stopping and starting points in the series.
Hornblende or pyroxene, further, may be pure silicate of lime
and magnesia, or iron and manganese may partially replace these
substances. Lime carbonate may be pure, or magnesia may
replace the lime in any proportion. 'These illustrations are
sufficient to indicate the reason of the great simplicity of rock
masses as regards their chief constituents, and that whatever
may be the composition of a mass within nature's limits, and
whatever may be the conditions of its origin, the probabilities
are that it will be formed essentially of one or more of a half
a dozen minerals in some of their varieties.

10 THE MTNEKALS CONSTITUTING EOCKS

But however great the adaptability of these few minerals may
be, they are, nevertheless, subject to very definite laws of chem-
ical equivalence. There are elements which they cannot take
into their composition, and there are circumstances which retard
their formation while other minerals may be crystallizing. In
a mass of more or less accidental composition it may, there-
fore, be expected that other minerals will form in consider-
able numbers, but minute quantities. It is customary to speak
of those minerals which form the chief ingredients of any
rock, and which may be regarded as characteristic of any
particular variety, as the essential constituents, while those
which occur in but small quantities, and the presence or
absence of which does not fundamentally affect its character, are
called accessory constituents. The accessory mineral which pre-
dominates, and which is present in such quantities as to be
recognizable by the unaided eye, is the characterizing accessory.
Thus a biotite granite is a granite composed of the essential
minerals quartz and potash feldspar, but in which the accessory
mineral biotite occurs in such quantities as to give a definite
character to the rock.

The minerals of rocks may also be conveniently divided into
two groups, according as they are products of the first consoli-
dation of the mass or of subsequent changes. We thus have :

(1) The original or primary constituents, those which formed
upon its first consolidation. All the essential constituents are
original, but, on the other hand, all the original constituents
are not essential. In granite, quartz and orthoclase are both
original and essential, while beryl and zircon or apatite, though
original, are not essential.

(2) The secondary constituents are those which result from
changes in a rock subsequent to its first consolidation, changes
which are due in great part to the chemical action of percolat-
ing water. Such are the calcite, chalcedony, quartz, and zeo-
lite deposits which form in the druses and amygdaloidal cavities
of traps and other rock?.

Below is given a list of the more important rock-forming
minerals, arranged as above indicated. Although these are
sufficiently described as regards their chemical and crystal lo-
graphic properties in any of the mineralogies, it has seemed
advisable to devote some space here to a reconsideration of
those most prominent as rock constituents, in order that the

EOCK-FOKMING MINERALS

individual characteristics of the rocks of which they form a
part may be better understood. In passing them in review
we will also note briefly the characteristic alteration and de-
composition products to which they may give rise, though the
cause of such changes must be left for another chapter.

A. ORIGINAL MINERALS.

1. Quartz.

2. The Feldspars.
2 a. Orthoclase.
2 b. Microcline.
2 c. Albite.
2d. Oligoclase.
2 e. Andesine.

2 /. Labradorite.
2g. Bytownite.
2h. Anorthite.

3. The Amphiboles.

3 a. Hornblende.
3 6. Tremolite.

3 c. Actinolite.
3 d. Arvedsonite.

3 e. Glaucophane.
3/. Smaragdite.

4. The Monoclinic Pyroxenes.

4 a. Malacolite.
4 6. Diallage.

4 c. Augite.
4d. Acmite.

4 e. JEgerite.

5. The Khombic Pyroxenes.

5 a. Enstatite (Bronzite).
5 6. Hypersthene.

6. The Micas.

6 a. Muscovite.
6 6. Biotite.
6 c. Phlogopite.

7. Calcite (and Aragonite)

8. Dolomite.

9. Gypsum.

10. Olivine.

11. Garnet.

12. Epidote.

13. Zoisite.

14. Andalusite.

15. Staurolite.

16. Scapolite.

17. Elasolite and Nepheline.

18. Leucite.

19. Sodalite.

20. Hauyn (nosean).

21. Apatite.

22. Menaccanite.

23. Magnetite.

24. Hematite.

25. Chromite.

26. Halite (common salt).

27. Fluorite.

28. Graphite.

29. Carbon.

30. Pyrite.

1. Quartz.

1 a. Chalcedony.

16. Opal.

1 c. Tridymite.

2. Albite.

SECONDARY MINERALS.

3. The Amphiboles.
3 a. Hornblende.
36. Tremolite.
3 c. Actinolite.
3d. Uralite.

THE MINERALS CONSTITUTING KOCKS

4. Muscovite (Sericite).

5. The Chlorites.
5 a. Jefferisite.
5 b. Ripidolite.
5 c. Penninite.

5 d. Prochlorite.

6. Calcite (and aragonite).

7. Wollastonite.

8. Scapolite.

9. Garnet.

10. Epidote.

11. Zoisite.

12. Serpentine.

13. Talc.

14. Glauconite.

15. Kaolin.

16. The Zeolites.

16 a. Laumontite.

16 &. Phrenite.
16 c. Thomsonite.
16 d. Natrolite.
16 e. Analcite.
16 /. Datolite.
16 g. Chabazite.
16 h. Stilbite.
16 i. Heulandite.
16 k. Phillipsite.
16 I. Ptilolite.
16m. Mordenite.
16 n. Harmotome.

17. Hematite.

18. Limonite.

19. Gb'thite.

20. Turgite.

21. Pyrite.

22. Marcasite.

Quartz. Composition: Pure silica, Si0 2 ; specific gravity 2.6;
hardness, 7. 1

This is one of the commonest and most widely distributed
minerals of the earth's crust, and forms an essential constituent
in a variety of eruptive and sedimentary rocks, such as granite,
quartz porphyry, liparite, gneiss, mica schist, quartzite, and
sandstones. In the granites, gneisses, and schists it occurs in
the form of irregular granules destitute of crystal outlines.

1 For convenience in determining minerals, the ' ' scale of hardness ' ' given
below has been adopted by mineralogists. By means of it one is enabled to
designate the comparative hardness of minerals with ease and definiteness.
Thus, in saying that serpentine has a hardness equal to 4, is meant that it is
of the same hardness as the mineral fluorite, and can therefore be cut with
a knife, but less readily than calcite or marble.

1. Talc: Easily scratched with the thumbnail.

2. Gypsum: Can be scratched by the thumbnail.

3. Calcite: Not scratched by the thumbnail, but easily cut with a knife.

4. Fluorite : Can be cut with a knife, but less easily than calcite.

5. Apatite: Can be cut with a knife, but only with difficulty.

6. OrtJioclase feldspar: Can be cut with a knife only with great difficulty

and on thin edges.

7. Quartz: Cannot be cut with a knife; scratches glass.

8. Topaz: Will scratch quartz.

9. Corundum: Will scratch topaz.
10. Diamond: Will scratch corundum.

/ M ^

<&*-'

QUAETZ 13

In the quartz porphyries and liparites it is found as a porphy-
ritic constituent, with well-defined crystal outlines, which may
however have become more or less obliterated through the cor-
rosive action of a molten magma. (See Fig. 3, PL 5.) In
the secondary rocks, quartzite and sandstone, the quartz occurs
as more or less rounded or irregularly angular grains without
crystal outlines, except it may be through a secondary deposition
of silica, as explained on p. 136. Quartz is the hardest and most
indestructible of the common constituents, and hence when rocks
containing it decompose and their debris becomes exposed to
combined chemical and mechanical agencies, it remains unaltered
to the very last, forming the chief constituent of beds of sand and
gravel, which in turn may become transformed into sandstones,
quartzites, or conglomerates.

Quartz is usually easily recognized, either under the micro-
scope or by the unaided eye, by its clear, colorless appearance,
irregular, glass-like fracture, hardness, and insolubility in any
acids but hydrofluoric. Under the microscope it appears in
clear, pellucid grains, often highly charged with minute cavities
filled with liquid and gaseous carbonic acid, the latter like the
bubble in a spirit level dancing about from side to side of its
minute chamber as though endowed with life.

As a secondary constituent quartz occurs, filling veins and
cracks in other rocks, and in the impure crypto-crystalline and
amorphous forms known as chalcedony, chert, flint, opal, hya-
lite, and agate is found as an infiltration product in the cavities
of many trappean rocks, in lenticular and oval concretionary
masses in limestones, and replacing the organic matter of wood
and other organisms. The name tridymite is given to a quartz
occurring in minute, usually microscopic, tablets in cavities in
volcanic rocks, particularly the more acid varieties. (See fur-
ther on p. 67.)

The Feldspars. Hardness, 5 to 7; specific gravity, 2.5 to
2.8. The feldspars are essentially anhydrous silicates of alu-
minum, with varying amounts of lime, potash, or soda, and
rarely barium. They have in common the characteristics of
two easy cleavages inclined to one another at an angle of 90,
or nearly 90 ; close relationship in optical properties; similarity
in colors, which vary from clear and transparent through white,
yellowish pink, and red, more rarely greenish, and often opaque

14 THE MINEEALS CONSTITUTING EOCKS

through impurities or decomposition; and lastly, a constant
intergradation in composition, as already noted on p. 9.

Nine varieties of feldspar are commonly recognized, which
on crystallographic grounds are divided into two groups: the
first, crystallizing in the monoclinic system, including ortho-
clase and hyalophane; and the second, crystallizing in the tri-
clinic system, including microcline, anorthoclase, and the albite-
anorthite series albite, oligoclase, andesine, labradorite, and
anorthite.

The Monoclinic Feldspars: Orthoclase (Sanidin), Potash Feld-
spars. Composition: K 2 Al 2 Si 6 16 = silica, 64.7% ; alumina,
18.4%; potash, 16.9%.

This is one of the commonest and most abundant of feldspars,
and forms an essential constituent of the acid rocks, such as gran-
ite, gneiss, syenite, and the orthoclase and quartzose porphyries ;
more rarely it occurs as an accessory in the more basic erup-
tives. Under the name Sanidin is included the clear glassy
variety of orthoclase occurring in Tertiary and modern lavas,
such as trachyte, phonolite, and the liparites.

As a rock constituent the potash feldspars are of primary im-
portance, imparting by their preponderance, not merely color
and important structural features, but on their decomposition
yielding the potash, valuable for plant food, and the material
kaolin so essential for porcelain ware. In the thin sections,
under the microscope, the orthoclase of the older rocks is often
quite opaque, or at least muddy, through impurities or incipient
kaolinization. In many eruptives it has been one of the first
minerals to separate out from the molten magma, and shows,
therefore, more or less well-defined crystallographic boundaries
is idiomorphic, to use a more technical term. A well-defined
zonal structure is frequently observed, which is due to inter-
rupted periods of growth, and to a gradual change in the char-
acter of the magma, whereby the outer zones are more or less
translucent or opaque from impurities. Twin structure is very
common after what is known as the Carlsbad law, and when
the crystals are of sufficient size is easily recognized by the
unequal reflection of the light from the two sides of a crystal
on a cleavage surface.

The Triclinic Feldspars. The chemical relationship exist-
ing between the triclinic feldspars is shown in the following
table :

THE TRTCLINIC FELDSPARS

8i0 2

A1 S 8

K,0

Na,0

CaO

Microcline

65.00%

18.00%

17.00%

Albite

68.00

20.00

12.00%

Oli^oclase . ...

62.00

24.00

9 00

5 00

Labradorite

53.00

30.00

4 00

13 00

Anorthite

43.00

37.00

....

20.00

Considering only the last four of these, as arranged, it will
be noted that they become gradually poorer in the acid element
silica, and richer in alumina and other bases; that is, they
become more basic. Also that albite carries some 12% of soda
and no lime; that oligoclase carries 9% of soda and 5% of lime;
labradorite but 4% of soda and 13% of lime, while anorthite,
the most basic of all, has no soda, and carries 20% of lime.
They have hence come to be known, respectively, as soda feld-
spar, soda-lime feldspar, lime-soda feldspar, and lime feldspar.
As a matter of fact, however, these varieties all grade into one
another, through the replacing power of the various elements,
and are regarded, not as true species, but rather as isomorphous
admixtures, forming what is known as the albite-anorthite series.

Their distinction, either in hand specimens by the unaided
eye, or in thin sections by the miscroscope, is a matter of con-
siderable difficulty, and as in addition to other characteristics
they have in common two eminent cleavages occurring at oblique
angles, it has become customary to group all under the general
term of plagioclase, a name derived from two Greek words signi-
fying oblique and fracture. We can then treat of the subject
under the heads of (1) microcline and (2) plagioclase.

(1) Microcline (Triclinic Potash Feldspar). As a rock con-
stituent, this feldspar is in every way identical with orthoclase,
from which it can be distinguished only in thin sections under
the microscope. Its composition, manner of occurrence, and
associations are those of orthoclase, and need not be repeated
here. Anorthoclase is a triclinic soda-potash feldspar of a form
closely resembling that of orthoclase and for all present purposes
may be regarded as orthoclase in which soda replaces a con-
siderable proportion of the potash.

(2) The Plagioclases. With the exception of albite the
plagioclases are all prominent and essential constituents of the

16 THE MINERALS CONSTITUTING BOOKS

basic eruptive rocks. As a rule they are recognizable only as
feldspars by the unaided eye, and recourse must be had to the
microscope or to chemical tests for their final determination.
Examined in thin sections and by polarized light, they show a
beautiful parallel banding in light and dark colors, which is
due to multiple twinning, the alternate bands becoming light
and dark in turn as the stage of the microscope is revolved.
When the crystals are of sufficient size, this twinning is some-
times evident in the form of fine straight, parallel bands, or striae,
but in rock masses, as already noted, recourse must be made to
microscopic methods. In form the plagioclase of effusive rocks
is most frequently slender and elongated, lath-shaped, as com-
monly described, and often with very perfect crystal outlines.
In the norites and gabbros, they are short and stout, imparting
a granular character to the rock. They occur frequently in
crystals of two or more generations, of which the earlier formed
are usually the largest and best developed. The common forms
are described in detail below:

(1) Albite, or soda feldspar, occurs as an original constituent
in many granites in company with orthoclase ; it is* also found
in gneiss, the crystalline schists, and not infrequently in diorite,
phonolite, trachyte, and other eruptives. (2) Oligodase, soda-
lime feldspar, occurs like albite in the acid eruptives like gran-
ite and quartz porphyry, but is also a common constituent of
diorite, and the younger eruptives such as trachyte, the aride-
sites, and more rarely of the diabases. It is also a constituent
of many gneisses. (3) Labradorite, or lime-soda feldspar, is a
prominent constituent of the basic eruptives of all geological
ages, such as the norites, diabases, and basalts. Andesine and
bytownite are closely allied varieties of similar habit, the first
being a trifle more acid, and the second more basic than labra-
dorite. (4) Anorthite, or lime feldspar, is also a prominent and
important constituent of the basic eruptives, and has been found
in meteorites and terrestrial peridotites.

On account of their abundance and wide distribution, as well
as on account of the character of their decomposition products,
the feldspars are to be considered the most important of rock
constituents. As it is from the debris of the older feldspathic
rocks that have been made up a large proportion of all the
sedimentaries of more recent date, so too it may be claimed
that from the decomposition of this feldspathic constituent has

THE DECOMPOSITION OF FELDSPAKS 17

been derived a large share of the salts of potash, lime, and soda,
as well as aluminous silicates which form so essential a portion
of the soils. The method of feldspathic decomposition as com-
monly understood is given on p. 223.

This decomposition usually manifests itself by a whitening
of the mass, accompanied by opacity and a general softening,
whereby it falls away to loose powder unless confined. As seen
in thin sections under the microscope, the decomposition goes
on most rapidly along lines of cleavage, naturally attacking the
outer portions first, so that the crystals show fresh unaltered
cores surrounded by opaque and "muddy" borders. In cases
where the feldspars carry iron this usually makes its presence
known by a reddening or browning of the mass, due to oxida-
tion. In presence of abundant carbonic acid, the liberated iron
may be carried away in solution and the color remain unchanged.

Daubree, who submitted feldspathic fragments to trituration
in revolving cylinders of stone and iron, found that in all such
cases not merely were the particles worn down to the condi-
tion of fine silt, but that an actual decomposition took place, as
well, as shown by the presence of alkalies in the form of soluble
silicates in the water with which the cylinders were partially
filled.

The production of kaolin through feldspathic decomposition
has become so well recognized that it is customary to speak
of this form of decomposition as kaolinization, a term which we
shall have frequent cause to use. 1

It should be noted that orthoclase, though so frequently found
muddied and impure, apparently in an advanced stage of de-
composition, does not in reality decompose so readily as the
plagioclase (soda-lime) varieties. This fact has been noted by
Lemberg, 2 who states that the apparent decomposition may be
due to physical causes, as disintegration, inclusions of some
easily decomposable silicate, or to originally water-filled cavities
the contents of which have been absorbed through the formation
of secondary hydrous silicates.

1 The statement by Kosler (Neues Jahrbuch fur Min. Geol. u. Petrog.,
Beilage Band, Vol. 2, 1902) to the effect that kaolinization is never due
to weathering, but is a deep seated process, finds little confirmation in
America.

2 Zeit. Deut. Geol. Gesellschaft, 35, 1883.
3

18 THE MINEEALS CONSTITUTING EOCKS

Leucite. Composition: Silica, 55.0%; alumina, 23.5%; pot-
ash, 21.5%.

Leucite occurs as an original and essential constituent of
many volcanic rocks, but is not an abundant mineral except in
rare instances. Its chief interest, from the present standpoint,
lies in its high percentage of potash which must become available
as plant food on decomposition. Leucite is a common constitu-
ent of certain lavas of Vesuvius, and it is not improbable that
this may account in part for the well-known fertility of the soils
of that region, though naturally climatic influence has much
to do.

Nepheline; Elaeolite. These names are given to what are
varietal forms of one and the same mineral. In composition
they are silicates of alumina, soda, and potash of the formula
(NaK) 2 Al 2 Si 2 8 = silica, 41.24; alumina, 35.26; potash, 6.46;
soda, 17.04.

Nepheline occurs in Tertiary and post-Tertiary eruptive rocks,
and is an essential constituent of phonolite, tephrite, and nephe-
linite. The variety elaeolite occurs only in older rocks, and is an
essential constituent of elaeolite syenite.

Both nepheline and elaBolite gelatinize readily with hydro-
chloric acid, and the powdered rock when treated on a glass slide
with this acid yields abundant microscopic cubes of sodium
chloride. This is one of the easiest of microchemical tests for
the determination of the mineral. Nepheline occurs as a rule
in well-defined short and stout hexagonal prisms, which in longi-
tudinal sections show up as short, colorless rectangular areas.
Elaeolite differs in being more opaque and occurring in less well-
defined, more granular forms. When occurring in sufficient
abundance in a rock mass it is readily recognized by its char-
acteristic greasy appearance. The mineral undergoes a ready
alteration, giving rise to zeolitic minerals and on ultimate de-
composition through weathering, yielding a rich and fertile soil.
(See p. 196.)

The Amphiboles. Composition: Two principal varieties are
recognized. (1) Non-aluminous, consisting mainly of the meta-
silicates of magnesium and calcium, with 55 to 59% of silica,
21 to 27% of magnesia, 11 to 15% of lime, and small pro-
portions of protoxides of iron and manganese. Under this head
are included the white, gray, and pale green, often fibrous forms,
as tremolite, actinolite, and asbestos. (2) Aluminous, contain-

THE AMPHIBOLES

ing silica, 40 to 51% ; magnesia, 10 to 23 % ; alumina, 6 to 14% ;
lime, 10 to 13% ; ferrous and ferric oxides, 12 to 2Q%. Here
are included the dark green, brown, and black varieties.

The aluminous variety, common hornblende, is an original
and essential constituent of diorite, and of many varieties of
granite, gneiss, syenite, schist, andesite, and trachyte, and is
also present as a secondary constituent in many rocks, result-
ing from the molecular alteration of the augite. The non-
aluminous varieties occur in gneiss, crystalline limestone, and
other metamorphic rocks.

By the unaided eye, or by means of blowpipe tests alone it is
often impossible to distinguish the minerals of this group from
the pyroxenes. In the thin sections this distinction is, however,
a matter of comparative ease. Green fibrous hornblendes may
result from the molecular alteration of augite, and all varieties
are susceptible of alteration into chloritic and ferruginous prod-
ucts with the separation of calcite.

On decomposing, the amphiboles give rise to ferruginous and
aluminous or magnesian products. In the darker colored varie-
ties, the decomposition begins with hydration and the peroxida-
tion of the iron along lines of cleavage and fracture, whereby
the crystal becomes riddled with corroded areas filled with the
liberated iron in the form of hydrated sesquioxide.

When the disintegration is complete, the whole mass is con-
verted into an ochre-brown, earthy substance and ultimately
passes into a ferruginous clay. These chemical changes are indi-
cated in the following analysis of I. fresh, and II. decomposed
hornblende from Haavi on Fillejeld, Norway :*

ANALYSES OF FRESH AND DECOMPOSED HORNBLENDE

Silica .

4537

40.32

Alumina .

1481

,17.49

Iron protoxide ....
Manganese

8.74
1 50

Iron sesquioxide. .

18.26
2.14

Lim6

1491

6.37

Magnesia .

1433

9.23

Water . . .

8.00

99.66

100.81

Bischof 's Chemical Geology, Vol. 11, p. 354.

20 THE MINEEALS CONSTITUTING EOCKS

The most striking features of these analyses are (1) the
complete conversion of the protoxides into sesquioxides, (2) the
loss in lime and magnesia which have presumably been carried
away in the form of carbonates, and (3) the assumption of 8%
of water. As the dark aluminous and ferruginous hornblendes
are among the commonest and most wide-spread of minerals, it
is apparent from the above that they may have an important
bearing upon the color and physical qualities of the residual
clays; to which they thus give rise. The peroxidation of the
iron gives yellow, brown, or red colors, while the hydrated
aluminous silicate (clay) imparts tenacity. The final product
of such decomposition is, then, a ferruginous clay as already
noted.

The Pyroxenes. The rock-forming pyroxenes are divided
upon crystallographic grounds into two groups, the one ortho-
rhombic in crystallization, and the other monoclinic. All varie-
ties, when in good crystalline form, show in basal sections an
octagonal outline bounded by prismatic and pinacoidal faces,
with a well-defined cleavage parallel with the prism faces.
Chemically they are silicates of magnesia and iron with lime
/and alumina in varying proportions. They are hard, tough
minerals and have an important bearing upon the physical
properties of the rocks of which they form a part.

The Monoclinic Pyroxenes. Two principal varieties are
recognized. (1) Pyroxenes containing little or no alumina, and
composed of silica, 45.95 to 55.6%; lime, 21.06 to 25.9%; mag-
nesia, 13.08 to 18.5%, with sometimes varying quantities of iron
oxides and water. Under this head are included the lighter
colored varieties, malacolite, sahlite, and diallage. (2) Pyroxenes
containing alumina, and composed of silica, 49.40 to 51.50% ; alu-
mina, 6.15 to 6.70%; magnesia, 13.06 to 17.69%; lime, 21.88
to 23.80% ; iron oxides, 0.35 to 7.83%, with sometimes small
quantities of soda and water. Under this head are included
the darker varieties, augite and leucaugite.

The lighter colored, non-aluminous varieties, malacolite and
sahlite, are common in mica and hornblendic schists, gneiss,
and granite, though not always in sufficient abundance to be
noticeable to the naked eye. The foliated variety, diallage,
is an essential constituent of the rock gabbro, and is also
common in peridotites. The darker colored, aluminous vari-
ety, augite, is an essential constituent of diabase and basalt,

THE MICAS 21

and also occurs in many syenites, andesites, and other eruptive
rocks.

The aluminous varieties undergo alteration into chloritic and
ferruginous products, while the non-aluminous give rise to ser-
pentine, either process being attended by the separation of
free calcite.

The Orthorliombic Pyroxenes. These are essentially silicates
of magnesia and iron, the latter replacing the former in varying
proportions up to 25%. Two principal varieties are recognized,
the distinction being founded mainly upon their optical prop-
erties which seem to be affected very largely by the percentages
of iron. Enstatite is the theoretically pure magnesian silicate
of the formula MgSi0 3 , but which, as a matter of fact, usually
contains from 2 to 10% or more of iron. The highly ferruginous
varieties are known as bronzite, from their bronze-like lustre.
Hyperstkene contains from 10 to 25% of ferric oxide.

Both enstatite and hypersthene are common constituents of
basic igneous rocks, such as the gabbros, norites, and perido-
tites. Enstatite is also a common constituent of meteorites.
Both forms are liable to alteration, giving rise to serpentinous
pseudomorphs to which the name bastite has been given, and to
talcose and chloritic products. The general character of the
decomposition products of the pyroxenes, as well as the methods
by which the decomposition progresses, are in every way similar
to those of the amphiboles, and need not be further dwelt upon
here.

The Micas. There are several species of mica which are
prominent as rock constituents, the more important being the
white variety, muscovite, and the dark variety, biotite. Both
occur in platy forms, splitting readily into thin, elastic folia,
which in crystalline form are hexagonal in outline. The folia
are often bent and distorted, and the mineral frequently under-
goes alteration into a chloritic or sericitic product. ^The micas
exercise an important influence upon the rocks containing them,
on both color and structural grounds. Other things being equal,
the muscovite-bearing rocks are lighter in color than those carry-
ing biotite. If the mica plates are arranged in definite planes, the
rock assumes a schistose structure and splits more or less readily
into sheets an important feature from an economic stand-
point. Muscovite, or potash mica, a silicate of alumina and
potash, is a constituent of .many granites, gneisses, and schists,

22 THE MINERALS CONSTITUTING EOCKS

but is rarely met with in other rocks, and is wholly wanting in
the basic eruptives. Sericite is a silvery white, or greenish,
hydrous, secondary mica occurring commonly as an alteration
product from feldspar. Lepidolite, a lithia mica of a white or
faint pink color, is frequently found in pegmatitic veins in the
older rocks.

Biotite, the black iron mica, is a silicate of alumina, iron, and
magnesia, and is much more general in its distribution than is
muscovite. It undergoes alteration into chloritic and ferrugi-
nous products and is often an important feature in hastening
rock disintegration. Other black micas, sometimes distinguish-
able from biotite only by chemical means, are lepidomelane and
houghtonite. A pearl gray potash mica, phlogopite, is an im-
portant constituent of many limestones, as in northern New
York and adjacent portions of Canada.

All micas, owing to their eminently fissile structure, allow the
ready percolation of moisture, and hence, though in themselves
of difficult solubility, are elements of weakness in any stone of
which they may form a part. The characteristic form of de-
composition begins as in other silicate minerals, with hydration.
This in the dark varieties is accompanied by a higher oxidation
of the iron. The folige gradually lose their elasticity and crumble
away, the bases being removed in solution. The complete de-
composition of the micas is, however, brought about very slowly,
and almost any granitic soil, however thoroughly decomposed,
will, on washing, show small flakes of the mineral still remaining.
However rusty, too, these may appear, a little hydrochloric acid
cleans them up, showing remnant shreds still readily recog-
nizable. For some unexplained reason those granitic rocks
containing a considerable proportion of white mica are almost
invariably more friable and easily disintegrated than those con-
taining biotite.

Olivine (Chrysolite, Peridote). Composition: Silicate of
iron and magnesia, (MgFe) 2 Si0 4 .

This is an essential constituent of basalt, dunite, imburgite,
Iherzolite, and pikrite, and a prominent ingredient of many
lavas, diabases, gabbros, and other igneous rocks. It also occurs
occasionally in metamorphic rocks and is a constituent of most
meteorites. Olivine is subject to extensive alteration, becom-
ing changed by hydration into serpentine or talcose and chloritic
products, with the separation of free iron oxides. It occurs in

EP1DOTE

well-defined crystals and also in irregular grains, either singly or
grouped in peculiar clusters to which the name polysomatic has
been applied by Tschermak. The serpentinous alteration takes
place along the irregular curvilinear lines of fracture, and under
favorable conditions continues until the transformation is com-
plete. The following analyses by Helland, as quoted in Teall's
British Petrography, illustrate the simplicity of the chemical
changes which here take place :

ANALYSES SHOWING CHANGE OF OLIVINE TO SERPENTINE.

III

SiO 2

41.32%

42.72 %

43.48 %

AloO 3
FeoO*

0.28
239

0.06
' 225

CrO . . .

005

Trace

M<K)

5469

42.52

4348

H 2 O

020

13.39

13.04

98.93%

100.94%

100.00%

I. Olivine, Snarum, Norway. II. Serpentine derived from the same.
III. The theoretical composition of serpentine.

Aside from the assumption of some 13% of water, the princi-
pal change, as will be noted, is a loss in magnesia which as a
rule separates out as a carbonate. The iron, which existed as
protoxide, is further oxidized and crystallizes out along lines of
fracture as magnetite or hematite, or in the hydrous sesquioxide
form known as limonite. Through decomposition, a portion or
all of the silica may be set free as opal or chalcedony, the mag-
nesia going over to the condition of carbonate, and the iron
passing into various hydrated oxide forms such as are most
stable under the existing circumstances.

Epidote. Composition: Silica, 37.83%; alumina, 22.63%;
iron oxides, 15,98% ; lime, 23.27% ; water, 2.05%.

This mineral is a common constituent of many granites,
gneisses, and schists, especially the hornblendic varieties. It
is particularly abundant, however, as a secondary constituent
in basic eruptives, where it results from the alteration of the
original ferromagnesian constituents such as the augites, horn-
blendes, or micas. It is the presence of this mineral or a sec-

24 THE MINERALS CONSTITUTING ROCKS

onclary chlorite that gives the characteristic color to many of the
so-called greenstones (altered basalts, diabases, and diorites).

Calcite (Calcium Carbonate). Composition: CaCO 3 = Car-
bon dioxide, 44% ; lime, 56%. Hardness, 3.

This is an original constituent of many secondary rocks,
such as limestone and calcareous shales. It is the essential con-
stituent of most marbles, of stalactites, travertine, and calc-sinter.
The shells of foraminifera, brachiopods, crustaceans, and many
lamellibranchs and gasteropods are also of this material. As a
secondary constituent, resulting from the decomposition of other
minerals, it occurs filling wholly or in part cavities in rocks of
all ages.

The effervescence of the mineral when treated with a dilute
acid furnishes the most ready means for its detection. Under
the microscope it appears as colorless grains with faint irides-
cent polarization, and is best recognized by its cleavage and
characteristic twinning lines as shown in the figure on p. 142.
Being soluble in carbonated waters, it is liable to complete re-
moval, or leaves only its impurities behind as a mark of its decay.

Aragonite. Composition: CaC0 3 Carbon dioxide, 44%;
lime, 56%.

This mineral has the same chemical composition as calcite,
but differs in its crystalline form and specific gravity. It is
found with beds of gypsum and veins of ore, and also in stalac-
titic and stalagmitic forms. In small quantities it occurs as a
secondary product in many trap rocks and basalts, and is the
substance of which the shells of many gasteropods and lamelli-
branch mollusks are composed.

The mineral is distinguished from calcite by its crystallization
and cleavage. As a rock constituent it is comparatively unim-
portant. This form of calcium carbonate, as long ago pointed
out by Sorby, is less stable than calcite, and in many instances
where the substance has first crystallized in the orthorhombic
form aragonite, it is found to have undergone a molecular altera-
tion into calcite.

Dolomite. Composition: (CaMg)C0 3 Calcium carbonate,
54.35% ; magnesium carbonate, 45.65%. Hardness, 3.5-4.

This mineral, like calcite, is wide-spread, and forms extensive
masses which are of value as sources of building material. It
is distinguishable from calcite by its greater hardness, higher
specific gravity, and in being but slightly acted on by dilute

APATITE AND THE IRON OEES 25

acids. In itself the mineral is less susceptible to atmospheric
influence than calcite, yielding much less readily to decomposing
agencies of a purely chemical nature. Nevertheless, Roth 1 has
shown that in the weathering of dolomitic limestones the mag-
nesia is sometimes removed by leaching, in greater proportional
quantities than the more soluble lime carbonate.

Apatite. Composition: Phosphate of lime. Hardness, 5.

Apatite is an almost universal constituent of eruptive rocks,
both acid and basic, though as a rule present only in micro-
scopic proportions. In the granular limestones, schists, and other
metamorphic and vein rocks, it sometimes occurs in large crys-
tals or massive forms in such abundance as to be of value as a
source of mineral phosphate for fertilizing purposes. Though
present in but small amounts, apatite is an important constituent,
since it is the only common rock constituent containing the valu-
able element phosphorus.

THE IRON ORES

Under this head may conveniently be grouped the several
forms of iron oxides which occur as rock constituents, and which
from their opacity in even the thinnest sections, and occasionally
similarly in crystallographic outline, are separable with difficulty
by optical test alone.

Magnetite. Composition : FeO + Fe 2 3 = iron sesquioxide,
68.97%; iron protoxide, 31.03%.

This is a wide-spread and almost universal constituent of
eruptive rocks, occurring in the form of scattering, small, and
rather inconspicuous granules, which are characterized by a
complete opacity and bluish lustre. When of sufficient size to
be distinguished by the unaided eye, magnetite is recognized by
its brilliant lustre, weight, and its property of being readily
attracted by the magnet. Magnetite also occurs as a constituent
of metamorphic rocks and is sometimes found in large beds,
constituting a valuable ore of iron. Under continual alterna-
tions of heat and cold, moisture and dryness, it slowly decom-
poses, giving rise to hydrated sesquioxides which impart color
to the resultant sands and clays.

Menaccanite (Ilmenite or Titanic Iron). Composition:
(TiFe) 2 3 , a mixture in varying proportions of the oxides
of iron and titanium.

1 Chemische u. Allgemeine Geologie.

26 THE MINERALS CONSTITUTING ROCKS

This, like magnetite, occurs in scattering granules as an
original constituent of many eruptives, and also in micaceous
lamellar and vein-like masses in other rocks. This form of iron
ore is extremely refractory to atmospheric agencies and is to be
found scarcely, if at all, changed in the residuary materials
resulting from the breaking down of the rocks in which it origi-
nally formed a part.

Hematite (Specular Iron Ore). Composition: Anhydrous
sesquioxide of iron, Fe 2 O 3 = iron, 70.9% ; oxygen, 30.20%. H =
5.5-6.5.

This mineral occurs in varying proportions and under vary-
ing conditions in rocks of all ages. In the form of minute
scales of a blood-red color, it is found in granitic and other
eruptive rocks. It occurs, also, in large beds, forming a valu-
able ore of iron. In the amorphous condition, it may form the
cementing constituent of sand-stones, and is the cause of the
red color of many rocks, both clastic and metamorphic, and of
soils as well. The usual coloring constituent is, however, limon-
ite or turgite, as noted below. The specular and massive forms
are best recognized by opacity, brilliant, black, metallic lustre,
and red streak.

Limonite (Brown Hematite). Composition: Hydrous ses-
quioxide of iron, H 6 Fe 2 O 6 + Fe 2 3 iron sesquioxide, 85.6% ;
water, 14.4%. H = 5-5.5.

This is a common constituent of rocks of all ages, but is
wholly secondary, resulting from the decomposition of ferrugi-
nous silicates, sulphides, and anhydrous oxides. As a coloring
constituent it is more abundant than hematite, and like it forms
a valuable ore of iron. (See p. 101.) Turgite (Fe 4 H 2 O 7 ) in the
form of a brilliant red ochreous material is also a common con-
stituent of soils and clays resulting from the decomposition of
siliceous rocks, and is presumably, like limonite, a product of
the spontaneous hydration of the iron salts thus set free. (See
further under Color of Soils, p. 374.)

Pyrite (Iron Pyrites). Composition: Iron disulphide, FeS 2
= iron, 46.7%; sulphur, 53.3%. H = 6-6.5.

Two principal forms of iron disulphide occur in nature, alike
in chemical composition, but differing in forms of crystalliza-
tion and in density. The one is common pyrites which crys-
tallizes in the isometric system, and is easily recognized by its
strong brassy yellow color and hardness. Its usual form of

PYEITE AND CHLORITE 27

occurrence is that of cubes, the corners and edges of which may
be more or less modified by secondary planes, and in concre-
tionary masses. The second form, marcasite, also called gray,
white, or cockscomb pyrites, is of lighter color, inferior hardness
and density, and crystallizes in the orthorhombic system. Its
most common form of occurrence is that of irregular concre-
tionary masses.

Both forms of pyrite are susceptible to oxidation when exposed
to atmospheric agencies, though of the two the pyrite proper
is much the more refractory. There is a difference in the char-
acter of the products arising from the decomposition of the two
compounds, pyrite yielding limonite and perhaps free sulphur,
while marcasite, under the same conditions, yields ferrous sul-
phate, though it may also yield limonite. The sulphate of
iron, resulting from pyritiferous decomposition, is, if present in
quantity, injurious to plant growth. This fact was well illus-
trated some years ago on the west front of the National Museum
at Washington. Several large masses of iron sulphide, too large
for exhibition within the building, were placed here upon a
floor of cement bordered by a narrow strip of lawn. Under
the oxidizing influence of rain and air the sulphide became
slowly converted into sulphate which was washed down upon
the cement and thence into the soil, which it so poisoned as to
kill the grass roots and necessitate an entire resodding.

Chlorite (Viridite). Under the general name chlorite are
included several minerals occurring in fibres and folia, closely
resembling the micas, from which they differ in their large per-
centage of water, and in their folia being inelastic. The three
principal varieties recognized are, ripidolite, penninite, and pro-
chlorite, any one of which may occur as the essential constitu-
ent of a chlorite schist. Chlorite as a secondary product often
results from and entirely replaces the pyroxene, hornblende, or
mica in rocks of various kinds, and also occurs filling wholly or
in part the amygdaloidal cavities of trap rocks. In this last form
it is frequently visible only with the microscope, and owing to
the difficulties in the way of an exact determination of its
mineral species is sometimes called viridite. It is this mineral
which gives the green color to a large share of the more or
less altered eruptives, like the diabases and diorites, the "green-
stones" of the older geologists.

Serpentine. Composition: A hydrous silicate of magnesium

28 THE MINERALS CONSTITUTING ROCKS

corresponding to the formula H 4 Mg 3 Si 2 O 9 = silica, 44.1%; mag-
nesia, 43.0% ; and water, 12.9%.

The prevailing color is green, though often spotted and
streaked; hence the name from the Latin serpentinus, serpent-
like. It has a somewhat greasy lustre and may be cut with a
knife, having a hardness of about 4 of the scale. The mineral
is always secondary, resulting mainly from the hydration of
magnesium or lime magnesium silicates. (See further on p. 107.)

Glauconite. This name is given to a somewhat variable
compound consisting essentially of silica, iron, alumina, and
water, with smaller amounts of potash, and incidentally lime,
magnesia and soda. The prevailing color is green, and as it
occurs in single granules or granular aggregates, it is com-
monly known as greensand. It is always a secondary mineral,
and has been formed and is still forming on many shallow sea-
bottoms which receive fine sediments derived from the breaking
down of siliceous crystalline rocks. (See under Greensand
Marl, p. 116.)

The Zeolites. Under this head are grouped a number of
minerals alike in being hydrous silicates of alumina with vary-
ing percentages of lime, potash, and soda. They are altogether
secondary minerals, resulting from chemical changes taking
place in pre-existing rocks, and indicate the first or deep-seated
stages of rock decay. In a more or less perfect condition they
have been assumed to occur in soils, having been derived from
the rocks, or, as is contended by some authorities, having formed
during the process of rock decomposition or in the soil itself.
It is thought possible that those constituents of a soil which
on analysis are found to be soluble, as the term is ordinarily
used, may, in part at least, have once existed as zeolites. Hence
their consideration in this connection is of importance.

Out of the 22 species of minerals classified as zeolites by
Dana in his System of Mineralogy there are but 11 which, on
account of their abundance or chemical composition, need con-
sideration here. The theoretical composition of these, as indi-
cated from a comparison of several to many analyses, is shown
in the accompanying table. In addition to the true zeolites are
included several other hydrous silicates closely related, both as
regards chemical composition and mode of occurrence, and which
in the present discussion cannot well be excluded.

GLAUCONITE AND THE ZEOLITES

COMPOSITION OF ZEOLITES

SILICA
(Si0 2 )

ALUMINA

(A1 2 8 )

LIME
(CaO)

BARIUM
(BaO)

POTASH
(K 3 0)

SODA
(Na,0)

WATER
(H0)

Ptilolite . . .
Mordenite .
Heulandite .

70.0
67.2
59.2

11.9
11.4

16.8

4.4

2.1
9.2

2.4
3.5

0.8
2.3

10.5
13.5
14 8

Phillipsite . .
Harmotome . .
Stilbite . . .
Laumontite .

48.8
47.1
67.4
51.1

20.7
16.0
16.3
21.7

7.6

7.7
11.9

20.6

6.4
2.1

1.4

16.5
14.1
47.2
15 3

Chabazite . .
Analcite . . .

47.2
54.5

20.0
23.2

5.5

....

6.1
14 1

21.2
8 2

Natrolite

47.4

26.8

16 3

9 5

Thomsonite .
Prehnite . . .

36.9
43.7

31.4
24.8

11.5

27.1

....

6.4

13.8
4.4

Apophyllite . .

53.7

25.0

....

5.2

....

16.1

See further on p. 363.

IV. THE PHYSICAL AND CHEMICAL PROPERTIES

OF ROCKS

1. STRUCTURE

In considering the structure of rocks it will facilitate mat-
ters to do so under two heads: (1) the macroscopic (or mega-
scopic) structures, or structures visible -to the unaided eye
(macros, from Greek word /mx/aos, signifying large) ; and (2)
microscopic structures, or those visible only with the aid of the
microscope.

1. Macroscopic Structures. From a structural standpoint
all rocks may be classified closely enough for present purposes,
under the heads of: (1) Crystalline, (2) vitreous or glassy,
(3) colloidal, and (4) clastic or fragmental. Of the first of
these, ordinary granite or crystalline marbles are good types,
being made up wholly of crystal aggregates, without interstitial
amorphous or fragmental material. The term crystalline gran-
ular, or granular crystalline, is applied to such as have a dis-
tinctly granular structure, as do many of the granitic rocks.
Vitreous or glassy structures are found only among igneous
rocks, and are due to a cooling of the molten magma too rapidly
for the production of crystals. Obviously, as the rate of cooling
in rock masses must be extremely variable, so one finds all
intermediate stages between the completely glassy and the crys-
talline forms. To these intermediate stages such names as felsitic
and microlitic are given, the precise meaning of which will be
stated under the head of microscopic structures. Rocks origi-
nating as chemical deposits, and which have since undergone no
structural changes, often present a jelly or glue like structure
known as colloidal. Such are exemplified in the siliceous sinters
from the Yellowstone National Park, and by various other
forms of silica, and occasionally by serpentines.

A clastic or fragmental structure is found only in secondary
rocks, and is the result of a breaking down or disintegration of
pre-existing rocks, and a reconsolidation of their particles with-
out crystallization. There are many minor points of structure,

PLATE 2

FIG. 1. Quart/ pori)hyry showing porphyritic structure.
FIG. "2. Quartz porphyry showing flow structure.

MACEOSCOPTC STKUCTUEE 31

some of which are common to all of the primary groups above
mentioned, while others are limited to one or more. Rocks
which are made up of distinct grains, whether crystalline or
fragmental, are spoken of as granular; when the structure be-
comes too fine and dense for macroscopic determination it is
spoken of as compact, though there is no reason why the term
should not equally well be applied to the coarser grained rocks
in which the individual grains are closely cohering without
interstices. The term massive is applied to such igneous rocks
as show no signs of bedding or stratification, while limestones,
sandstones, and such other rocks as are arranged in more or
less parallel layers are described as stratified. (See Fig. 1,
PI. 12.) The name foliated or schistose is given to a rock in
which the arrangement of the constituent minerals in parallel
planes is sufficiently marked to cause it to split in one direction
more readily than in any other. Not infrequently the quartzes
or feldspars occur in lens-shaped forms about which curve the
hornblende or mica folia as shown in Fig. 2, PI. 12. As ex-
plained elsewhere, this structure may be due to original deposi-
tion or may be secondary. In eruptive rocks a fiuidal or fluxion
structure is not uncommon, as shown in Fig. 2, PL 2, and is
due to the onward flowing of the mass while gradually cooling
and passing into a solid state. Eruptive magmas at the time of
their extrusion contain more or less moisture, which, being
highly heated, expands whenever sufficient force is developed
to overcome the pressure of the overlying mass. In this way
are formed innumerable cavities or bubbles, comparable to the
cavities caused by carbonic acid from the yeast in well-raised
bread. Such cavities are called vesicles, and the rocks contain-
ing them are vesicular (Fig. 2, PI. 3). By the subsequent
actiorj. of percolating waters these cavities may become filled
with a variety of secondary minerals, among which chalcedony,
epidote, calcite, and various zeolites are not uncommon. Such
refilled cavities are called amygdules, from the Greek word
afjivySaXov, an almond, in allusion to their shape, and the rocks
containing them are therefore described as amygdaloidal. The
upper part of a lava flow sometimes cools in peculiar ropy
forms like the slag from a smelting furnace. Such forms are
known as slaggy. (See Fig. 1, PI. 3.)

When a rock consists of a compact, glassy, or fine and evenly
crystalline ground-mass, throughout which are scattered larger

32 PHYSICAL AND CHEMICAL PEOPEETIES OF EOCKS

crystals, usually of feldspar, the structure is said to be porphy-
ritic (Fig. 1, PI. 2). This structure is quite common in granite,
but is not particularly noticeable, owing to the slight contrast in
color between the larger crystals and the finer ground-mass. It
is most noticeable in such effusive eruptives as the quartz por-
phyries, in which the ground-mass is exceedingly dense and com-
pact and of a black or red color, while the large feldspar
crystals are white and stand out in very marked contrasts.
This structure is so striking in appearance that rocks possess-
ing it in any marked degree are popularly called porphyries,
whatever may be their mineral composition. The name is said
to have been originally applied to certain kinds of igneous rocks
of a reddish or purple color, such as the celebrated red porphyry
or "roseo antico" of Egypt. The word is now used almost
wholly in its adjective sense, since any rock may possess this
structure whatever its origin or composition may be.

Glassy rocks on cooling sometimes have developed in them
a series of concentric cracks whereby a broken surface shows
numerous rounded or globular bodies with an onion-like shell.
This structure, which may be visible only with a microscope, is
known as perlitic. It is not uncommon in glassy forms of
trachyte and liparite.

Glassy and felsitic eruptives, particularly of the liparite and
quartz porphyry groups, frequently show spherulitic masses of
all sizes, from microscopic to several inches or even feet in
diameter, usually with a well-defined radiating structure, which
are due to incipient crystallization. Such are known as spheru-
lites, and hence rocks in which they occur are described as
spherulitic. 1

Concretionary forms may be developed in rocks either as
primary or secondary structures. Many of the forms thus de-
veloped are peculiarly deceptive, and it may not be out of place
to enter into a discussion of their nature and origin with some
detail.

On genetic grounds such may be divided into two groups:
(A) Primary concretions, formed contemporaneously with the
rocks in which they are found, and (B) secondary concretions,

1 The structure and origin of these forms has been worked out in detail by
Whitman Cross. Bull. Philosophical Society of Washington, Vol. XT, 1891,
pp. 411-462.

PLATE 3

FIG. 1. Basalt showing slag.sry structure. FIG 2. Basalt showing vesicular structure.

MACROSCOPIC STEUCTURE 33

or those which are due to segregating influences acting subse-
quently to the formation of the rocks of which they now form a
part. All are due to that peculiar and little understood ten-
dency which atoms or molecules of like nature so often manifest
in concreting or gathering in amorphous masses or concentric
layers about some foreign body which serves as a primary point
of attachment. The extreme development of this tendency is
seen in crystallization. Under primary concretions may be
included the flint and chalcedonic nodules found in chalk and
the older limestones, the material of which was in part derived
from the siliceous remains of radiolaria and sponges. Such
sometimes occur in the form of lenticular nodules with or with-
out an appreciable concentric structure, lying in parallel layers
or beds, continuous for long distances. Clay iron stone, an
impure carbonate of iron, occurs characteristically in this form.
These latter often crack on drying and consequent shrinkage,
the cracks extending from within outward. In these cracks cal-
cite is subsequently deposited, whereby the nodule is divided up
into septa of a white or yellowish color. On being cut and
polished, these often form beautiful and unique objects. To
such the name septarian nodule is commonly given. (See Fig.
2, PI. 8.) The carbonate of lime in inland lakes and seas may
become deposited in the form of thin pellicles about a minute,
perhaps microscopic nucleus, forming small, spherical bodies
which, when ultimately consolidated into beds, give rise to the
oolitic and pisolitic limestones. (See p. 125.)

All primary concretions are not, however, chemical deposits;
but, rather, aggregates of mineral particles in a finely frag-
mental condition.

Such are the clay concretions which are found in the beds
of streams and lakes, and which may so closely simulate animal
forms as to be very misleading. The manner in which concre-
tions of this nature are formed was shown in a very interesting
manner a few years ago during the progress of the work of filling
in the so-called Potomac flats, on the river front at Washington,
District of Columbia. For the double purpose of raising the
flats and deepening the channel, gigantic pumps were employed
which raised the sediment from the river bottom in the form
of a thin mud and forced it through iron pipes to the flats,
where it flowed out, spreading quietly over the surface. The
material of this mud was mainly fine siliceous sand and clay
4

34 PHYSICAL AND CHEMICAL PROPEBTIES OF BOCKS

intermingled with occasional fresh-water shells and plant debris.
As it flowed quietly from the mouth of the pipe and spread out
over the surface, the clayey particles began quickly to separate
from the siliceous sand in the form of concretionary balls, which
in the course of a very short time would grow to be several
inches in diameter. Such, owing to the rapidity of their for-
mation, contained a large amount of sand and shells, though
clayey matter predominated.

In crystalline rocks concretionary structure is less commonly
developed. Cases such as shown on Plate 7 are unique, and
in the case of the orbicular diorite of great interest on account
of the beauty of the stone and its adaptability for small orna-
mentation.

Concretionary structure of a secondary nature may be de-
veloped through the process of weathering. Thus, by the oxi-
dizing action of meteoric waters percolating through a porous
sand or sandstone, included nodules of iron disulphide (pyrite)
may be converted into an oxide which gradually segregates in
zones about the original nodule. This oxide, by its cementing
action, binds the grains together in the form of a hard crust,
leaving the central portion, formerly filled by pyrite, either
empty or occupied by loose sand. 1 A zonal banding closely simu-
lating concretionary structure is common in rocks more or less
weathered and decomposed, but which is due not to original dep-
osition or crystallization of mineral matter about a centre, but
rather to the weathering of jointed blocks, the various chemical
agencies acting from without inward.

A botryoidal structure is not uncommon among rocks and
minerals of chemical origin. It is, as a rule, confined to such
as are amorphous or radiating crystalline aggregates of a single
mineral, as chalcedony or the hematite iron ores. (See Fig.
1, PL 8.)

A brecciated structure, produced by the presence of angular
fragments in a finer ground, is of common occurrence among
fragmental rocks, but is more rare among the crystallines. It
is sometimes produced in volcanic rocks by the imbedding in the
still pasty magma of angular fragments of previously consoli-
dated material, as shown in Fig. 2, PI. 4. Columnar structure,
though comparatively common as the structure of a geological

1 See On the Formation of Sandstone Concretions, Proceedings U. S. Na-
tional Museum, Vol. XVII, pp. 87, 88.

PLATE 4

FIG. 1. Chert breccia cemented by zinc blende.

FIG. 2. Felsite breccia formed of felsitic fragments embedded in a matrix of the same
composition.

MICROSCOPIC STRUCTURE 35

body, is rarely developed among the constituents of the rock
itself. The columnar structure of many lavas and dike rocks
has already been alluded to: occasionally the mineral constitu-
ents of some secondary rocks are arranged after this manner.
A cavernous or cellular structure is developed through the re-
moval by solution of some constituent or the weathering out of a
fossil. As an original structure it occurs in many rocks of chem-
ical origin as the stalagmitic deposits in caves, travertines, etc.

A laminated or banded structure, due to the arrangement of
the constituents in parallel layers or bands, is common in rocks
of sedimentary origin, particularly in sandstones and shales.

2. Microscopic Structures. Many, if not indeed the ma-
jority, of rocks are so fine grained and compact that little of their
mineral nature or structural features can be learned from exami-
nation by the unaided eye. This difficulty made itself apparent
very early in the history of geological science, and to it is per-
haps due, more than to any other single cause, the apparent
crudities and fallacies of the early workers. As long ago as
1663, the microscope had been to some extent utilized for the
examination of minerals; but its application to the study of
rocks remained long unrecognized, though early in the nineteenth
century Cordier and others utilized it in the study of rocks in
a pulverized condition. It was not until about 1850, when the
subject was taken up by H. Clifton Sorby of England, that the
possibility of studying rocks in thin sections under the micro-
scope began to be appreciated. Even then the idea failed to
bear its legitimate fruits until transplanted to German soils,
where, under the fostering care of Professor Zirkel of Leipzig,
it soon began to yield an abundant harvest; and to-day the
branch of the science of geology known as microscopical pe-
trography hold a prominent place in all the leading universities,
both domestic and foreign. The efficiency of the method is
based upon the fact that every crystallized mineral has cer-
tain definite optical properties; i. e., when cut in such a way as
to allow the light to pass through it, will act upon this light in
a manner sufficiently characteristic to enable one working with
an instrument combining the properties of a microscope and
stauroscope to ascertain at least to what crystalline system it
belongs, and in most cases by studying also the crystal outlines
and lines of cleavage the mineral species as well. To enter
upon a detailed description of the method by which this is done

36 PHYSICAL AND CHEMICAL PEOPEETIES OF ROCKS

would be out of place here, since it involves the polarization of
light and other subjects which must be studied elsewhere. The
reader is referred to any authoritative work on the subject of
light, and to Professor J. P. Iddings's translation of Professor
Eosenbusch's work on optical mineralogy. 1

The method of study is of value, not merely as an aid in
determining the mineralogical composition of a rock, but also,
and what is often of more importance, its structure and the
various changes which have taken place in it since its first
consolidation. Rocks are not the definite and unchangeable
mineral compounds they were once considered to be, but are
rather ever-varying aggregates of minerals, which even in them-
selves undergo structural and chemical changes almost without
number. It is a common matter to find rock masses which may
have had originally the mineral composition and structure of
diabase, but which now are mere aggregates of secondary prod-
ucts, such as chlorite, epidote, iron oxides, and kaolin, with
perhaps scarcely a trace of the unaltered original constituents;
yet the rock mass retains its geological identity, and to the
naked eye shows little, if any, sign of the
changes that have gone on. These and
other changes are in part chemical and in
part structural or molecular. A very
common mineral transformation in basic
rocks is that from augite to hornblende.
This takes place merely through a molec-
ular readjustment of the particles, where-
by the augite, with its gray or brown col-
ors and rectangular cleavages, passes by

uralitic sta ^ es over into a ^ reen horn -

blende, a mineral of tho same chemical
composition, but of different crystallographic form. The trans-
formation in an incomplete state is shown in the accompanying
figure, in which the central, nearly colorless portion with rectan-
gular cleavage represents the original augite, while the outer dot-
ted portion with cleavage lines cutting at sharp and obtuse angles
is the secondary hornblende. This change is due to slow and

1 Microscopic Physiography of Rock-making Minerals, Wiley & Son, New
York. See also Professor A. Harker 's Petrology for Students.

MICBOSCOPIC STRUCTURE 37

gradual pressure exerted upon the rock masses, the final result
being a rock of entirely different type and structure from that
which originally cooled from the molten magma. The change
such as above described is further alluded to in the chapter on
metamorphism.

This science of microscopic petrography, as it is technically
called, has also been productive of equally important results in
other lines. As an instance of this may be mentioned the dis-
covery that the structural features of an igneous rock are de-
pendent, not upon its chemical composition or geological ,age,
but upon the conditions under which it cooled, portions of the
same rock varying from holocrystalline granular through por-
phyritic to glassy forms. To this fact allusion has already been
made.

The general subject of the microscopic structure of rocks of
various kinds will be discussed more fully in describing the
rocks themselves. Nevertheless, as in describing these struc-
tures it has become necessary to use sundry technical terms, it
will be well to refer to them briefly here.

When a rock is made up wholly of crystalline matter, it is
spoken of as holocrystalline; when, however, it shows interstitial
glassy or felsitic matter, it is hypocrystalline. Eocks wholly
without crystalline secretions are amorphous. The glassy, or
felsitic matter occupying the interstices of the other constitu-
ents is spoken of as the base. This base, together with the
microlites and smaller crystallizations of the second generation,
is called the ground-mass; such may be made up of microlites
small needle-like crystals imperfectly developed when it is
called microlitic, or of a dense aggregate of quartzose, felds-
pathic and other materials, when it is known as felsitic. The
larger crystals developed in a glassy, felsitic, microlitic, or finely
granular microcrystalline ground-mass are called phenocrysts.
When a mineral in a rock shows good crystal outlines, having
been uninfluenced in its growth by the proximity of other
minerals, it is called idiomorphic: when, however, its outline is
due not to crystallographic forces, but to interference to the
action of external forces it is allotriomorphic. Many rocks
show indications of two or more periods of crystallization, in
each of which minerals of the same species may be developed.
Thus in a molten magma the augites may begin to form under
such conditions that for some time their growth is unimpeded

38 PHYSICAL AND CHEMICAL PROPERTIES OF ROCKS

and they take on large and well-developed forms. After a time,
owing to changed conditions, their growth is stopped, and the
rock solidifies with a new crop of smaller and less perfectly
developed forms. It is customary to speak of such a mineral
as occurring in crystals of two generations. In the case above
described, the first developed form the porphyritic constituents,
the phenocrysts, while the latter formed are a part of the ground-
mass. Vitreous or glassy rocks may show, under the microscope,
minute, hair-like or rod-shaped forms, representing the first
stages of crystallization, but in which the process was arrested
before they were sufficiently developed to render possible an
accurate determination of their mineral nature. Such are termed
crystallites; those in drop-shaped or globular forms being called
gldbulites, the rod-shaped ones belonites, and the twisted, hair-
like forms trichites.

The wide variation in microstructure in rocks of essentially
the same chemical composition, but which have cooled under
the varying conditions indicated above, is shown in Figs. 1 to
4 of PL 5, Fig. 1 being a holocrystalline type, and Fig. 4 one
almost completely glassy, the first being a deep-seated rock, and
the last a surface lava flow. Intermediate structures are often
produced through a beginning of crystallization at certain pe-
riods, after which, and while a portion of the magma was still
fluid, it was brought under such conditions as resulted in a
more rapid cooling, the final result being a glassy, or micro-
crystalline rock with scattering porphyritic crystals, or pheno-
crysts. It has in many instances happened that, subsequent to
the formation of these earliest products of crystallization, a
second elevation of temperatures has taken place whereby the
magma has eaten into or corroded them, as is the case with
the quartz crystal shown in the centre of Fig. 3 of PL 5.

Inasmuch as this study by the microscope involves the prepa-
ration of thin sections, a brief description of the methods pur-
sued may well be given here. The fact that a chip of rock,
however dense, can, without breaking, be ground so thin as
to be transparent, may at first seem strange, but in reality it
is readily accomplished. The work requires only patience and
the skill which comes from practice. A small chip of rock,
about the size of a nickel five-cent piece, is broken off with a
hammer, care being taken to get it as thin as possible without
fracturing. One side of this is then ground flat and smooth by

PLATE 5

FIG. 1. Microstructure of granite.

FIG. 2. Microstructure of micropegmatite.

FIG. 3. Microstructure of quartz porphyry.

FIG. 4. Microstructure of porphyritic obsidian.
FIG. 5. Microstructure of trachyte.
FIG. f>. Microstructure of serpentine.

MICROSCOPIC STRUCTURE 39

rubbing it in water and emery on a smooth, cast-iron plate.
Toward the close of the process fine flour of emery is used,
as the final surface must be very smooth and free from scratches.
This chip is then cemented smooth side down on a piece of
ordinary double-thick window glass, a convenient size being
about 2X1 inches, the cementing material being Canada balsam
which has been evaporated to the extent that, when cold, it is
sufficiently hard to hold firmly, is not at all sticky, but yet is not
so hard as to be brittle. The exact degree can only be learned
by experience; a hardness such as to be barely indented by the
thumb nail will be found about right. This operation of ce-
menting is best done by means of a thin iron plate laid hori-
zontally on a support and heated not too hot by a lamp beneath.
The glass with the balsam upon it is heated to the right tem-
perature, the balsam being fluid and free from bubbles. The
rock chip, heated sufficiently to expel all moisture, is then pressed
firmly into the balsam, in such a way as to exclude air bubbles,
and brought within as close contact with the glass as possible.
It is then removed from the iron plate and allowed to cool,
when the grinding process is resumed, the glass plate serving
merely as support for the film of stone and something for the
fingers to hold by. Being transparent, the worker can see just
how the grinding is progressing without continually stopping to
examine. When sufficiently thin, usually from T -V<r to m-
of an inch, the film is remounted as follows: While on the
thick glass on which it was ground, it is thoroughly washed
with a brush an ordinary tooth-brush serves well to get
rid of all particles of emery and other dirt that may adhere. It
is then washed in alcohol to get rid of the old hard balsam, which
is usually quite dirty from mud produced in grinding. Fresh
mounting slips and clean cover glasses being ready, the first is
laid upon the warm iron plate with a couple of drops of balsam
in the centre, and allowed to heat until it begins to smoke.
Care must here be exercised, as, if heated too much, the balsam
becomes hard and brittle, and if too little, the mount is sticky
from the balsam which constantly oozes from under the cover.
The thick glass, with its film of stone still adhering, is likewise
laid upon the warm iron plate, and a drop of fresh balsam placed
upon the film. This is then gently heated, and the cover-glass,
first warmed, gently laid upon it one edge placed in position
and lowered gradually in such a manner as to force out any acci

40 PHYSICAL AND CHEMICAL PROPERTIES OF ROCKS

dental air bubbles and finally pressed flat down against the

stone film. The film itself, if sufficiently warmed, no longer ad-
heres to the thick glass, and may be removed
to the clean slip for its final mounting. This
is best accomplished by taking up the thick
glass by means of a pair of forceps and push-
ing cover-glass and film together, with a
needle point set in a handle, off into the
balsam on a new slide. The cover-glass here
serves merely as a support for the thin film
during the process of transferring. Without
it there is danger of breakage. When fairly
transferred, the new slide is removed from
the hot plate, the cover pressed close down
against the film, adjusted in proper position
and allowed to cool. The superfluous balsam
may be then removed with a hot knife and

the section finally washed in alcohol. Thus completed, it forms

the "thin section" of the petrologist.

2. THE SPECIFIC GRAVITY OF ROCKS

The term specific gravity is used to designate the weight of
any substance when compared with an equal volume of distilled
water at a temperature of 4 C. This property is therefore
dependent upon the specific gravity of its various constituents
and their relative proportions. The exact or true specific
gravity of a rock may be obscured by its structure. Thus an
obsidian pumice will float upon water, buoyed up by the air
contained in its innumerable vesicles, while a compact obsidian
of precisely the same chemical composition will sink almost
instantly. This property of any subject is spoken of as the
apparent specific gravity in distinction from the actual com-
parative weight, bulk for bulk, of its constituent parts, which
could in the case of a pumice be obtained only by finely pul-
verizing so as to admit the water into all its pores. Inasmuch
as the structural peculiarities of any igneous rock as will be
noted later are dependent upon the condition under which it
cooled, it is instructive to notice that a crystalline aggregate
has a higher specific gravity, i. e., a greater weight, bulk for

THE CHEMICAL COMPOSITION OF ROCKS

bulk, than does a glassy, non-crystalline rock of the same chem-
ical composition. The property is therefore dependent upon
chemical (and consequently mineral) composition and struc-
ture, and as a very general rule it may be said that among the
igneous rocks those which contain the largest amount of silica
are the lightest, while those with a comparatively small amount,
but which are correspondingly rich in iron, lime, and magnesian
constituents, are proportionately heavy.

3. THE CHEMICAL COMPOSITION OF EOCKS

This varies naturally with their mineral composition. It is
customary to speak of sedimentary rocks as calcareous, sili-
ceous, ferruginous, or argillaceous, accordingly as lime, silica,
iron oxides, or clayey matter are prominent constituents.
Among eruptive rocks it is customary to speak of those show-

(1) STRATIFIED ROCKS

KIND

SPECIFIC
GRAVITY

COMPOSITION

Calcareous:
Limestone
Dolomite

2.6 to 2.8
2.8 to 2.95

Carbonate of lime.
Carbonate of lime and magnesia.

Siliceous:
Gneiss ... ...

2 6 to 2.7

Same as granite.

Siliceous sandstone . .
Schist . .

2.6

2 6 to 2 8

Mainly silica.
60 to 80 per cent silica.

Clay slate (argillite)

2.5

Mainly silica and silicate of
aluminum.

(2) ERUPTIVE ROCKS

KIND

SPECIFIC GRAVITY

PER CENT SILICA

Acidic group:
Granite

2 58 to 2.73

77.65 to 62.90

Liparite

2 53 to 2.70

Obsidian

2 26 to 2.41

78.06 to 67.61

Obsidian pumice
Intermediate group:

Floats on water.
2.73 to 2.86

72.20 to 54.65

Trachyte

2 70 to 2 80

64 00 to 60 00

2.70 to 2.90

62.00 to 50.00

2.54 to 2.79

66.75 to 54.73

Basic group:
Diabase

Basalt

1 2.75 to 2.95

50.00 to 48.00

Peridotite . ...

3.22 to 3.29

42.65 to 33.73

Peridotite (meteorite) ....

3.51

37.70

42 PHYSICAL AND CHEMICAL PROPERTIES OF ROCKS

ing, on analysis, upwards of 60% silica as acidic, and those
showing less than 50%, but rich in iron, lime, and magnesian
constituents, as basic. The extremes, as will be noted, are rep-
resented by the rocks of the granite and peridotite groups.

A series illustrating the above-mentioned properties may be
arranged as on p. 41. "With the eruptive rocks only the silica
percentages are here given. The results of the complete chem-
ical analysis of each variety are given further on, in the pages
devoted to their description.

4. THE COLOR OF ROCKS

The color of a rock is dependent upon a variety of circum-
stances which may all be generalized under the heads of min-
eral and chemical composition and physical condition. Iron
and carbon, in some of their forms, are the common coloring
substances and the only ones that need be considered here.
The yellow, brown, and red colors, common to fragmental rocks,
are due almost wholly to free oxides of iron. The gray, green,
dull brown, and even black colors of crystalline rocks are due
to the prevalence of silicate minerals rich in iron, as augite,
hornblende, or black mica. Rarely copper, manganese, and
other metallic oxides than those of iron are present in sufficient
abundance to impart their characteristic hues. As a rule, a
white or light gray color denotes an absence of an appreciable
amount of iron in any of its forms. The amber, bluish and
black colors of many rocks, particularly the limestones and slates,
are due to the prevalence of carbonaceous matter.

Among siliceous crystalline rocks the more basic are, as a rule,
of a darker color than the acid varieties, the color being due to
the fine grain and predominance of dark iron-magnesian sili-
cates, such as hornblende, augite, or black mica, or their chloritic
alteration products. The red or pink color sometimes occurring
in granitic rocks is due to the predominance of red or pink
feldspars, which in their turn owe their color to the presence
of iron.

Many feldspar-bearing rocks owe their color to the physical
condition of this important constituent. Thus with rocks like
the norite of Keeseville, New York, and the Quincy, Massa-
chusetts, granite, the dark color is largely due to the fact that
the feldspar is clear and glassy, allowing the light rays to pene-
trate and become absorbed. The beautiful chatoyant play of

THE COLOE OF BOCKS 43

colors sometimes shown By labradorite-bearing rocks like those
of northern New York and of Norway is apparently caused by
a separation of the individual crystals along cleavage lines, into
thin, transparent plates which reflect and partially polarize
the light that would otherwise penetrate and become absorbed.
Through weathering, such feldspars undergo a further physical
change, becoming soft and porous, and no longer allowing the
light to penetrate, but wholly reflecting it, causing the stone to
appear white. These white feldspars, as has been very neatly
expressed by the late Dr. Hawes, bear the same relation to the
glassy forms as does the foam of the sea to the water itself, the
difference in color being in both cases due to the changed physical
condition. Indeed, the color of rocks, as may be imagined, is
not constant, but liable to change under varying conditions.
Rocks black with carbonaceous matter will fade to almost white-
ness on prolonged exposure, owing to the bleaching out of the
coloring materials. Rocks rich in magnetite or free iron oxides,
protoxide carbonate, or sulphides, or in highly ferruginous
silicate minerals, are likewise liable to a change of color, be-
coming yellowish, red, or brown, through oxidation of the fer-
ruginous constituents. (See p. 243.) Translucent, nearly color-
less rocks or minerals, as those made up of crystals of calcite
or selenite, will on exposure become nearly opaque and snow-
white, owing to purely physical causes, as already noted in the
case of the feldspars. (See further in chapter on weathering.)

The cause of the color variations in certain rocks and min-
erals is, however, a matter concerning which it will not do, as
yet, to speak too decidedly. Analysis of a mineral may show
the presence of metallic oxides, but it does not necessarily fol-
low that whatever color the mineral may have is due to or in any
way related to these oxides. Thus the writer has shown 1 that
the onyx marbles (travertines) of Arizona and Mexico may
vary from pure white to green, and from yellow through brown
to red, without appreciable change in the actual amounts of
iron though there may be a change in the form of combination.
In the white and green varieties the iron exists as a carbonate ;
in the yellow, red, and brown varieties as a more or less hydrated
sesquioxide. Certain dark amber and bright rose-colored va-
rieties from California, and the Californian Peninsula, show,
however, no iron or other of the usual metallic coloring con-

1 Annual Keport U. S. National Museum, 1893, p. 558.

44 PHYSICAL AND CHEMICAL PROPEKTIES OF BOCKS

stituents, but burn perfectly white when submitted to high
temperatures and yield volatile organic compounds. The fact
that serpentines so frequently contain small traces of chromium,
early gave rise to the opinion that it was to this element that
was due the characteristic green color of the mineral. The
writer has elsewhere 1 described serpentines of a beautiful oil
yellow and deep green color which, however, contain not a
trace of chromium or manganese, but only iron, which in this
case is in combination as a silicate. (See p. 106.)

These color characteristics are of greater importance than
may at first appear, particularly from an economic standpoint.
One of the first essentials in a rock designed for architectural
use should be permanency of color. Deleterious changes are
particularly liable to occur in stone taken from below the water
level, where, protected from oxidation, or from variations in
temperature. Certain of the Ohio sandstones are of a blue-
gray color below the water level, but buff above, where the
included iron sulphides and protoxide carbonates have been
acted upon by oxidation. The student should early make
himself acquainted with these characteristics, as in the field it
is as a rule only the more or less weathered surfaces that pre-
sent themselves for inspection. This subject is again referred
to in the chapter on rock weathering.

Lustre as a property of rocks does not, owing to their com-
plex nature, possess the same value as a determinative charac-
teristic as among minerals. Certain of the more compact and
homogeneous varieties possess lustres which may be described
as vitreous, greasy, pearly, metallic, or iridescent. The meaning
of such terms is sufficiently evident, and the subject need not
be further dwelt upon here.

The fracture, or manner of breaking of any rock, is dependent
more upon structure than upon chemical or mineral ogical com-
position. Many fine and evenly grained crystalline or frag-
mental rocks break with smooth, even surfaces, and are described
as having a straight or even fracture. Others break with shell-
like concave and convex surfaces, and are said to have a con-
choidal fracture. Still others are splintery, hackly, or shaly,
words the meaning of which is sufficiently evident without their
being described in detail.

1 On the Serpentine of Montville, New Jersey, Prcc. U. S. National
Museum, 1888, p. 105.

V. THE MODE OF OCCURRENCE OF ROCKS

It is ordinarily assumed that the earth owes its present form
to having originated from a mass of incandescent vapor, and
passed, by gradual cooling and consequent condensation, from
gaseous through pasty or fluidal, and all intermediate stages
to its present condition. This, in brief, is the hypothesis of
Kant, and seems most readily to account for the facts as we
now know them. As to the character of the rock masses result-
ing from this primary cooling, little is known. Reasoning from
analogy, it seems safe to assume that they resembled the slags
from a smelting furnace, or some form of modern lavas, more
nearly than any other rock masses of which we have knowledge.
Whatever may have been their nature, they have long since
been obscured by rocks of secondary origin, or become so altered
through "dynamic and incidental chemical agencies as to be no
longer recognizable.

The oldest rocks of which we now have knowledge belong
to the group of gneisses and crystalline schists. They are as
a rule highly siliceous rocks, though frequently including con-
siderable thicknesses of crystalline limestone. They contain no
traces of what can be referred beyond doubt to an organic origin,
but from their banded or foliated structure, so closely simulating
bedding, they have in the past been considered as metamorphic ;
that is, as rocks laid down as sediments and crystallized by the
complex processes comprehended under the term metamorphism.
Rocks of this type, according to Dana, first appeared in North
America in the wide V-shaped area extending from Labrador
southwesterly to the Great Lakes, and thence northwesterly to
the Arctic regions. This area has since been added to by the
folding and crumbling processes incident to the formation of
the Appalachian and Rocky Mountain systems. Concerning the
geographical distribution of these rocks, as they now appear
exposed, nothing need be said here. They seem to form, as
has been stated, the actual floor of the continents upon which
all later deposits have been laid down, and through which and

46 THE MODE OF OCCURRENCE OF EOCKS

into which have been extruded and intruded the great variety
of igneous rocks which form so conspicuous a feature in many
a mountainous region. In order to properly understand that
which is to follow, a little space may well be devoted to a
consideration of the manner in which these rock masses occur,
so far as exposed to investigation.

Several varieties of igneous rocks, and particularly the gran-
itic types, occur in the form of immense oval or rounded masses,
protruded into overlying materials which dip away on all sides ;
such forms are ordinarily designated as bosses.- (PL 1.) It is
a form common to granite, gabbros, norites, etc. A laccolith
is a somewhat similar form due to the welling up of a magma
through a comparatively small vent, but which, instead of com-
ing to the surface, spread out laterally into dome-shaped masses
between the sheets of the overlying strata. When the intruded
matter has been so forced into or between overlying bedded
rocks as to appear like more or less regularly defined beds, they
are known as sheets or sills. Such, as a rule, may be distinguished
from superficial lava flows by their like condition of compact-
ness along both upper and lower contacts, surface flows being
more or less vesicular along the upper portions, owing to the
expansion of their included moisture. The name dike is given
to an eruptive mass of varying width included between well-
defined walls, and occupying a fissure or fault in previously
consolidated rocks. Such are inclined at all angles with the
horizon, and are usually of very moderate width, but may ex-
tend for miles. The dikes in any one region will frequently
be found to belong to one or more well-defined systems, each
system occupying fissures essentially parallel with one another.
Any one dike may remain comparatively uniform in width for
long distances, excepting when split up into smaller dikes. At
times, dikes may be traced to the parent mass a boss or lacco-
lith from which they radiate with more or less regularity.
The name volcanic neck or plug is given to the cylindrical mass
which results from the congealing of that portion of the lava
which remains in the volcanic vent when eruption ceases.
Through the erosion of the matter composing the cone of a vol-
cano, such are sometimes left exposed owing to their superior
hardness, forming very striking features of the landscape. The
general name lava is applied to any igneous rock, regardless of
geological age or mineral composition, which has been poured

IGNEOUS ROCKS 47

out on the surface of the earth in a molten condition. Such
are characterized by less perfect crystallization and a more
slaggy and vesicular structure than the deep-seated rocks. A
columnar jointing, due to cooling, is by no means uncommon,
particularly among basaltic lavas, although it is by no means
confined to them.

But a comparatively small proportion of the rocks composing
the superficial portions of the earth's crust the portions with
which we are more or less familiar are eruptive. They are
rather what are known as secondary rocks; that is to say, they
are rocks made over from these so-called primary rocks, which
we have been just discussing, by processes which will be described
later.

Any rock mass, be it eruptive or otherwise, lying exposed at
or near the surface of the ground finds itself subjected to a
multitude of disintegrating and decomposing agencies, which
are described more in detail under the head of rock weathering.
Leached and decomposed by meteoric waters, disintegrated by
heat and frost, or the mechanical action of waves and currents,
the rock masses slowly succumb, their materials being in part
removed in solution, or as debris mechanically transported by
every wind, rain, or running stream, down the slopes into
the valleys, and from the valleys into the seas. This debris, in
various stages of coarseness and fineness, to which we give
the name of bowlders, gravel, sand, or silt, undergoes by these
transporting agencies a system of assorting more or less com-
plete, and is carried to distances dependent upon its weight and
the force of the transporting agent. It requires no geological
or other special training to enable one to understand that the
force being the same, the finer and lighter materials will be
carried farthest, and that all must be deposited when the force
shall be expended. Consider, then, for purpose of illustration,
a stream flowing from a mountainous region and emptying
itself into the sea. Materials falling by gravity from the moun-
tain slopes, or washed by spasmodic rains into the stream, are
transported certain distances, according to the strength of the
current. For present purposes, it is sufficient to consider only
those portions which are transported quite to the mouth of the
stream and dumped into the sea. But as the water leaves its
narrow channel there is an almost instant diminution of the force
of its current, and consequent carrying power. As a result, it

48 THE MODE OF OCCUKEENCE OF EOCKS

begins to deposit its load, the coarsest and heaviest first, and
the finer materials further from the shore, the very finest, an
impalpable silt it may be, remaining suspended until the very
last. There will thus be formed a bed, or series of beds of vary-
ing thickness, of gravel, sand, and clay, the coarsest at the
bottom and nearest the shore, and the finest and last the most
remote.

But the streams vary from time to time in their carrying
capacity, and the action of the waves and tides together with the
dissolved salts, exert a modifying action, whereby this process
of sedimentation, as it is called, may not be quite so simple as it
first appears. 1 Enough has, however, been said to show that
beds of detritus laid down in this manner must occur in ap-
proximately horizontal layers, and that the layers may vary
greatly in the coarseness and fineness of their materials, as well
as in their mineral character. But there are still other processes
of sedimentation than the purely mechanical methods described
above. All natural waters contain more or less mineral matter,
of which lime is the more abundant. Through the secreting
power of marine animals, this lime is taken up in the form of
a carbonate to form shells and calcareous skeletons of molluscs,
corals, and other forms of marine life. On the death of the
secreting animal, the calcareous material is left to accumulate
in a more or less fragmental condition, forming thus the material
of the coral islands, and to a considerable extent the beds of lime-
stone the world over. The expression, to a considerable extent,
is used for the reason that it is doubtful if all of our limestones
are of purely animal origin ; in many a true chemical precipita-
tion plays a not unimportant part. This is especially true of
the oolitic varieties, and the fact is readily apparent when one
studies such in detail. Consider a shallow sea-bottom on which
are gradually accumulating in a finely divided condition the
fragmental remains of calcareous organisms of any kind. By
the undulatory action of the waves these are kept in almost
constant motion, though it may be but gently rolling from side
to side. Owing to evaporation, or a too rapid accumulation of
the lime for it to be abstracted by the lime-secreting animals,
the water becomes supercharged with this constituent, which is
then precipitated in the form of a thin pellicle around the most

1 See Conditions of Sedimentary Deposition, by Bailey Willis, Journal of
Geology, 1893, p. 476.

BEDDED OB STRATIFIED ROCKS 49

available nucleus, in this case the grains of calcareous sand upon
the bottom. Thus are gradually built up beds of no inconsid-
erable thickness, such as the well-known Carboniferous oolitic
limestones of Indiana and Kentucky. The microscopic structure
of stones of this class is shown in Fig. 7 on p. 105.

Rocks which were laid down in the manner just described,
whether composed of inorganic particles or fragmental materials
from marine and fresh water organisms, are designated as
sedimentary. They occur in more or less well-defined beds or
strata, and hence are spoken of as bedded or stratified. Owing to
the fact that they have in most cases been deposited in com-
paratively shallow water, they retain the superficial markings
made upon them by waves and other agencies prior to their final
consolidation.

Such naturally lie approximately horizontally where not sub-
sequently disturbed by earth movements. The earth's crust,
however, is by no means in a state of stable equilibrium, but,
being subjected to continuous stress or compressive force, is
often broken, crushed, or folded, and crumpled to an extra-
ordinary degree. The name fault is applied to the profound
fractures made by these movements, which, inclined at various
angles to the horizon, may extend for miles. Usually the rocks
on one side of a fault will be found to have sunk down, while
those of the other remain stationary or are raised, producing
thus an inequality of surface that may assume mountainous
proportions. Most mountain ranges, in fact, are due to a com-
bination of faulting and folding processes. It not infrequently
happens that the masses of rock, sliding over one another along
a line of fault, produce smooth or striated and often highly pol-
ished surfaces, to which the name slickensides is given. Such
are particularly noticeable among serpentinous rocks, being ap-
parently due to motion generated in the mass by increase in
bulk incident to its conversion into serpentine. 1 The name vein
is given to rock masses of chemical origin, deposited along pre-
viously existing fractures which may or may not be true faults.
By some authorities the name is also made to include the smaller
injections of igneous rocks. Such are here classed under the
head of dikes. It is customary to divide the veins into two
classes: (1) the mineral veins, in which the materials have

1 See On the Serpentine of Montville, New Jersey, Proc. U. S. National
Museum, 1888, p. 105.
5

50 THE MODE OF OCCUEEENCE OF BOCKS

been deposited from aqueous solution or sublimation between
the walls of a fissure; and (2) segregation veins, in which the
component materials have crystallized or segregated out of the
still unconsolidated, pasty, or colloidal rock. It is not always
possible to decide to which of the two classes a vein may be-
long, but as a rule the mineral (or fissure) veins are separated
by sharp and well-defined walls from the country rock, and
show a comb or banded structure. The segregation type is less
distinctly marked, the vein material being welded to the enclos-
ing rock, or seemingly passing into it by gentle gradations.

The unconsolidated materials, as sands and gravels, occur
not only in regularly bedded or stratified forms, but also in
hillocks and ridges to which special terms are applied. The
loose material washed down the mountain slopes by ephemeral
streams, and deposited at the mouth of gorges, may assume the
form of "a conical mass of low slope descending equally in all
directions from the point of issue." To such forms Gilbert has
given the name of alluvial cones. The material of these cones
varies in size from the finest powder to angular rocks weighing
many tons. It exhibits no regular bedding or stratification, but
coarse and fine debris are mingled in endless variety. There is a
well-marked gradation, however, to be seen as one travels from
the apex of a cone toward its periphery. At the apex it is com-
posed mostly of coarse, angular material, with fine silt-like clays
filling the interspaces, while toward the periphery the fine ma-
terial predominates. An alluvial fan differs in having greater
width in proportion to its thickness and in showing signs of
stratification. The name talus is given to the accumulations of
debris at the foot of rocky cliffs. Such are composed of angular
fragments, large and small, which have fallen from the cliffs
above. The name dune is given to the rounded hills of wind-
blown sand common in arid regions and on windy shores.
These are naturally of moderately fine and quite uniformly
assorted materials. In form and position they are ever chang-
ing, like drifts of snow, but are usually much steeper on the
leeward than on the windward sides. The character of the
material of which they are composed is most commonly sili-
ceous sand.

The names kame, esker, osar, and horseback are given to ridges
and mounds of sand and gravel deposited by the melting ice of
the glacial epoch. Drumlin is the name given to the peculiar

CLASTIC MATERIALS 51

low, gently and smoothly sloping lenticular hills composed of un-
assorted glacial debris, and which are common in eastern Massa-
chusetts and other glacial regions. The general name moraine
includes the heterogeneous materials brought down by glaciers
and ultimately deposited in undulating hills and ridges on their
final disappearance. (See further under The Regolith, p. 287.)

PART II

THE KINDS OF BOCKS

1 ' Some rin up hill and down dale knapping the chucky stones to pieces wi
hammers like sae many road-makers run daft. They say it is to see how the
warld was made." St. Eonan's Well.

REFERENCE has already been made to the fact that but
sixteen out of all the known elements enter into the compo-
sition of the earth's crust in other than comparatively minute
quantities. Also to the equally important fact that the com-
bination of these elements as represented in not above a score
of well-known mineral species go to make up the essential por-
tion of nearly all rock masses. Nevertheless, owing to the
variety of forms under which these rock masses occur, the vary-
ing conditions under which they originated, or the proportional
quantities of the various minerals which they may contain, we
find numerous and widely varying types of rocks, a satisfactory
consideration of which necessitates first some attempt at syste-
matic classification. It may be said at the outset, however, that
rock species, in the sense in which the word is used in mineralogy
and zoology, scarcely exist. It is true we may have, and par-
ticularly among igneous rocks, certain forms which on casual
inspection, or indeed on close inspection, with regard only to
limited geographical areas, seem to possess an individuality of
their own sufficient to entitle them to being considered as true
species. Yet, when we come to compare these with others, to
take into account their physical and chemical composition, their
structure and mode of occurrence, and above all to consider how
any rock varies within its own mass, and the still greater varia-
tion which may have been produced through alteration, it will
be seen that one form grades into another almost without limit,
that, indeed, no two are exactly alike, and that, were we to
attempt any hard and sharp lines of discrimination, our species-
making would practically resolve itself into an enumeration of
individual occurrences. This fact will become apparent as we

THE KINDS OF ROCKS 53

proceed, and further remarks on the subject may well be de-
ferred until we come to a discussion of individual groups. In-
deed, in the present, transitional state of knowledge regarding
the chemical and mineralogical composition of rocks, their struc-
tural features, and methods of origin, no scheme of classification
can be advanced that will prove satisfactory in all its details.
The older systems, which were made to answer before the intro-
duction of the microscope into geological science, are now known
to be founded upon what were in part false, and what have
proven to be wholly inadequate, data. This is especially true in
regard to eruptive rocks. The time that has elapsed since this
introduction has been too short for the evolution of a perfectly
satisfactory system ; many have been proposed, but all have been
found lacking in some essential particulars. To enter upon a
discussion of the merits and demerits of the various schemes
would obviously be out of place here, and the student is re-
ferred to the published writings of Naumann, Senft, Von Gotta,
Eichtofen, Vogelsang, Zirkel, Eosenbusch, Michel-Levy, Cred-
ner, Jukes Brown, and Geikie, as well as those of the American
Geologists, Dana, 1 Wadsworth, 2 and Iddings. 3 In the scheme
here presented the writer has aimed to simplify matters so far
as is consistent with observed facts, and has not hesitated to
adopt or reject 'any such portions of proposed systems as have
seemed desirable.

All the rocks forming any essential part of the earth 's crust
are here grouped under four main heads, the distinctions being
based upon their origin and structure. Each of the main di-
visions is again divided into groups or families, the distinctions
being based mainly upon mineral and chemical composition,
structure, and mode of occurrence. We thus have :

I. Igneous Rocks : Eruptive. Eocks which have been brought
up from below in a molten condition, and which owe their pres-
ent structural peculiarities to variations in conditions of solidi-
fication and composition. Having as a rule two or more essential

1 On Some Points in Lithology, Am. Jour, of Science, Vol. XVI, 1878, pp.
335 and 431.

2 On the Classification of Rocks, Bull. Mus. Comp. Zool. Harvard College,
No. 13, Vol. V; also Lithological Studies.

8 The Origin of Igneous Rocks, Bull. Philosophical Society of Washington,
1892. See also Quantitative Classification of Igneous Rocks, by W. Cross,
J. P. Iddings, L. V. Pirsson and H. S. Washington, Chicago, 1903.

54 THE KINDS OF EOCKS

constituents. In structure massive, crystalline, or glassy, ^er in
certain altered forms, colloidal.

II. Aqueous Rocks. Rocks formed mainly through the
agency of water, as (A) chemical precipitates or as (B) sedi-
mentary beds. Having one or many essential constituents. In
structure laminated or bedded; crystalline, colloidal, or frag-
mental; never glassy.

III. -ffiolian Rocks. Rocks formed from wind-drifted ma-
terials. In structure irregularly bedded ; f ragmental.

IV. Metamorphic Rocks. Rocks changed from their orig-
inal condition through dynamic or chemical agencies and which
may have been in part of aqueous, aeolian, or of igneous origin.
Having one or many essential constituents. In structure bedded,
schistose or foliated, crystalline or colloidal.

I. ROCKS FORMED THROUGH IGNEOUS AGENCIES.

ERUPTIVE

This group includes all those rocks which having been once
in a state of igneous fusion have been. forced upward and in-
truded into the overlying rocks in the form of bosses, laccoliths,
dikes, and sheets, or poured out upon the surface as lavas.

Concerning the source of eruptive rocks we are yet in igno-
rance. In times past they have been supposed by many to repre-
sent portions of the still unconsolidated interior of the earth.
The great variety of igneous rocks, the wide variation in chemical
composition as well as the apparent independence of closely
adjacent volcanoes, both in the matters of time of eruption and
character of erupted material, seem, however, to show that they
come not from a common reservoir, but from isolated and com-
paratively small areas where, for reasons not now well under-
stood, previously solidified rock masses have been so highly heated
as to become pasty or liquid; and then, through their own ex-
pansion, or that of included vapors, or by compressive forces
generated in the earth's crust, forced upward into the positions
they now occupy. The origin of igneous rocks belongs as yet
to the realm of speculation. We must here confine ourselves
more to their mineral and chemical nature, general physical
properties, and the conditions under which they occur.

Consider, then, a mass of molten rock material, to which
the term magma may be conveniently applied, and which by
the processes of eruption is forced upward toward the surface,
and let us dwell briefly upon the forms assumed by this magma
on cooling under the various conditions in which it finds itself.
It is obvious at the start that we can have actually to do with
but a comparatively limited portion of the products of any erup-
tion. If the molten material is poured out upon the surface and
there remains for inspection to-day, it is a necessary consequence
that the deeper-lying portions are obscured. If, on the other
hand, the superficial portions have been removed by erosion so
as to expose the deeply lying parts, we have only the latter for
study and observation. It is rare indeed that erosion has so

56 EOCKS FOKMED THROUGH IGNEOUS AGENCIES

acted on any one rock mass as to expose superficial and deep-
seated portions alike. In those regions of greatest geological
antiquity, erosion has removed more or less completely the
superficial parts and left for our inspection those portions of
a magma that at the time of eruption never reached the sur-
face, but cooled, it may be, under thousands of feet of super-
incumbent matter. Such rocks are as a rule more highly crys-
talline than those which flowed out upon the surface like the
modern lavas. Hence it is that from a very early period it
has been found convenient to divide the eruptive rocks into two
general groups: first, the intrusive or plutonic rocks; and sec-
ond, the effusive or volcanic rocks.

Although this classification has not been strictly adhered to
in the present work, a few words descriptive of the essential
distinctions between plutonic and effusive rocks will not be out
of place, since such distinctions, particularly in eroded regions,
afford the only criteria for discrimination as to the original
conditions under which a rock mass has been formed, and hence
are of value in the field.

As a general rule, it may be said that the structural features
of an eruptive rock depend upon the conditions under which
a magma has cooled, although undoubtedly the amount of
included vapor of water may .exert a powerful influence. As
Professor J. P. Iddings has well expressed it, "the chemical
differences of igneous rocks are the result of a chemical differ-
entiation of a general magma, and the structure of a rock is
dependent upon the physical conditions attending its eruption
and solidification." Now it is at once apparent that the greater
the depth below the surface at which a magma undergoes
solidification, or the greater its mass, the slower, more gradual,
will be that solidification, and hence the more complete and
coarser will be the crystallization. Hence the strictly plutonic
rocks are always holocrystalline. And, inasmuch as the weight
of the superincumbent matter has been such as to prevent the
expansion of included vapors to form steam cavities, so these
rocks are never vesicular or pumiceous, but compact and gran-
ular throughout. In cases where a plutonic rock has been
voided upward to fill a pre-existing rift in the form of a dike,
those portions of the magma coming in contact with the cold
walls on either hand will cool most quickly. Hence a dike is
most coarsely crystalline near the centre, and finer grained, per-

STEUCTUEAL FEATUEES OF IGNEOUS EOCKS 57

haps microcrystalline or even glassy, at the immediate contact.
These two phenomena may afford the only means of determining
whether a rock mass occurring in the form of a sheet between
sedimentary beds, is an intrusive or a contemporaneous lava
flow; whether it was injected between two previously existing
beds; or whether, as a lava flow, it was poured out over the
lower, first formed, after which the second was laid down upon
its surface. If formed as an intrusive sheet, one may expect to
find the rock more dense along both contacts, in addition to
which there may be more or less contact metamorphism of the
sedimentary beds from the action of the hot intruded material.
If poured out as a lava, on the other hand, contact metamorphism
and the dense, fine-grained portions will be limited to the lower
contacts, while, provided there had been no great amount of
erosion between the time of the pouring out of the molten mass
as a surface flow and the deposition of the newer sediments, the
upper portions will be less dense, perhaps even vesicular, sco-
riaceous, and glassy, while the sediments themselves, having
been laid down on cold consolidated material, remain wholly
unchanged. Such means of discrimination have been of the
greatest value in ascertaining the relative ages of portions
of the Triassic sandstones and associated traps in the eastern
United States.

The lava flows, cooling so much more rapidly than the plu-
tonic rocks, owing to their exposed position and relief from
pressure, often show but incipient forms of crystallization, or
are quite glass-like, as is the case with the obsidians of the
Yellowstone Park and elsewhere. Chemically these last are prac-
tically identical with granite, but they have cooled too quickly for
the forces of crystallization to act. Owing, further, to the ex-
pansive force of the included vapor of water, a constituent
of all lavas, these surface flows are' at times so filled with
cavities as to be quite pumiceous. The pumice purchased at the
drug-stores is but the froth from a lava which, had it cooled
slowly and under greater pressure, might have yielded a granite.

A common feature of the effusive or volcanic rocks is a flow
structure, sometimes visible only with the microscope, which
is due to a flowing movement of the magma while undergoing
consolidation. (See Fig. 2, PI. 2.) The characteristic structure
of effusive rocks is porphyritic, instead of granular, and repre-
sents two distinct phases of cooling and crystallization: (1) an

58 KOCKS FORMED THROUGH IGNEOUS AGENCIES

intratellurial period, marked by the crystallization of certain
constituents while the magma, still buried in the depths of the
earth, was cooling very gradually, and (2) an effusive period,
marked by the final consolidation of the material on or near
the surface. As this final cooling was much the more rapid,
the ultimate product is a glassy, felsitic, or sometimes holo-
crystalline ground-mass, enclosing the porphyritic minerals, or
phenocrysts, formed during the first or intratellurial stage. 1
Naturally the deeper-lying portions of an effusive mass, those
forming the under or lower portions of deep lava streams, will
be under conditions essentially similar to plutonic magmas, and
may cool so slowly as to become holocry stall ine. It is, more-
over, obvious that, could any superficial mass of erupted material
be traced back to its original deep-seated source, it would be
found to pass gradually from the volcanic to the plutonic type.
Hence it is that in the laboratory it is not always possible, from
the examination of the hand specimen or thin section only, to
determine to which of the two classes it may belong. We can
easily discriminate between the extremes, but there is a wide
intermediate zone where any such attempts are impracticable, as
indeed they are unnecessary. 2

Owing to a false impression which formerly prevailed relative
to the nature of the Palaeozoic effusives and those of Mesozoic,
Tertiary, and more recent times, dissimilar names have, in very
many instances, been applied to rocks which in other respects
than that of geological age are essentially one and the same.
Thus the name andesite is given to a rock in every respect
similar to porpkyrite, with the possible exception of a slight

1 Whitman Cross has shown that there are exceptions to this rule. See
The Laccolitic Mountain Groups of Colorado, 14th Ann. Rep. U. S. Geol.
Survey, pp. 231-235.

2 Intermediate between these plutonic and effusive types is still a third
phase of prevailing holocrystalline porphyritic structure, thus far found only
in dikes, which it has been proposed to group under the head of dike rocks
(gangesteine). Since such are but local phases of plutonic magmas, which
have been left to cool and crystallize between narrow walls, instead of
poured out upon the surface, such a subdivision seems scarcely called for
and as tending to still further confuse that which is already sadly con-
founded. The same may be said with reference to the now prevailing
tendency to give varietal names to every phase of magmatic differentiation,
and which has resulted already in such monstrosities of nomenclature as
ouachitite, monchiquite, yogoite, and absarokite.

KELATIONSHIP OF PLUTONIC AND IGNEOUS BOOKS 59

amount of devitrification the latter may have undergone owing
to its greater geological antiquity.

The name rhyolite likewise includes rocks with the structure
and composition of the older quartz porphyries, and though
intended by Richthofen to include only certain comparatively
modern acid lavas, has been shown by the late Dr. Williams 1
to be applicable to the pre-Cambrian lavas of the South Mountain
region of Pennsylvania. These and other names have, however,
become too firmly engrafted upon the literature to be too hastily
set aside, and may well be retained here.

The following table will serve to show the relationship, so
far as known, which exists between the plutonic rocks and
their effusive equivalents of whatever age. Thus the palago-
volcanic equivalents of the syenites are the quartz-free por-
phyries, and the neo volcanic equivalents, the trachytes. The
terms, acid, intermediate, and basic, as used, have reference to
the percentage amounts of silica, both free and combined, con-
tained by the representatives of the several groups. Rocks which,
like some of the peridotites, carry even less than 40% of silica
are sometimes spoken of as ultra basic.

INTRUSIVE OB PLUTONIC

EFFUSIVE OR VOLCANIC

Palaeovolcantc

Neovolcanic

Acid ]

65% -75% > Granites ....
Si0 2 j

r Syenites ....
Intermediate 1 --r , ,. .. >

Quartz porphyries . .
Quartz-free porphyries

Liparites(rhyolites)
Trachytes

Si0 2
Basic

.wepneime syenites;
(Foyaites) (
Diorites ....
' Gabbros, norites, {_
and diabases ("

Theralites . . .

Phonolites

Porphyrites ....
Melaphyrs and augitej
porphyrites j

(Not known) ....

Phonolites
Andesites
Basalts

jThephrites and
| basanites

40% to 55% H
Si0 2

Peridotites . .
Pyroxenites . . .
(Not known) . .
(Not known) . .
. (Not known) . .

Picrite porphyrites
(Not known) . . .
(Not known) . . .
(Not known) . . .
(Not known) . . .

Limburgites
Augitites
Leucite rocks
Nepheline rocks
Melilite rocks

1 Am. Jour of Science, Vol. XLIV, p. 482, 1892.

60 KOCKS FOEMED THEOUGH IGNEOUS AGENCIES

The researches of the past few years have made it evident
that eruptive rocks are to be satisfactorily studied only when
considered in their geographical as well as geological relation-
ships; that is to say, the eruptives of any particular region
must be considered with reference to their genetic relation to
others of the same region; such a relationship as is suggested
by regarding them all as but varying phases of a process of
differentiation from a common magma.

That such a relationship in many cases exists has apparently
been conclusively demonstrated by the work of Iddings 1 in the
Yellowstone Park, J. F. Williams 2 in Arkansas, Pirsson 3 in
Montana, and Brogger 4 in Norway, and many more recent
workers. The attempt at correlation of local types with those
of a somewhat similar nature at a distance is interesting and in-
structive, as showing on the whole a remarkable unity in nature 's
methods; but we must never lose sight of the fact that each
eruptive centre, throughout periods of activity interrupted it
may be by thousands of years, works out its own results accord-
ing to local conditions which may or may not harmonize with
those at distant points. It is possible to conceive that, could all
the rocks of any successive periods of eruption from a single
centre be once more relegated to a common magma, such might,
in its entirety, be an exact equivalent of others in remote portions
of the globe. The consolidated results from the cooling of ex-
truded portions of this magma may, however, show ever- varying
differences due to local conditions. In short, eruptive rocks must
be considered by geographic groups and with reference to
magmas.

Attempts at a satisfactory classification on other grounds
must prove invariably futile and tend only to retard, rather
than to promote, the science.

In the following pages the rocks are discussed in groups,
each group comprising all those having essentially the same
chemical composition, but differing (1) in degree of crystalliza-
tion, (2) in mode of occurrence, and (3) in geological age. In
all, there is, within certain limits, a considerable range in min-

l. Philos. Soc. of Washington, XII, 1892.
'Ann. Eep. Geol. Survey of Arkansas, Vol. II, 1890.
8 Bull. Geol. Soc. of America, Vol. VI, 1895.
*Die Eruptivgesteine der Kristianiagebeite, Christiania, Norway, 1894.

PLATE G

iwff

FIG. 1. Liparite, nevadite form.
FIG. 2. Liparite, rhyolite form.

FIG. 3. Liparite, obsidian form.
FIG. 4. Liparite, pumiceous form.

THE GEANITE-LIPAEITE GKOUP

eral composition, or at least in the relative proportion of the
various essential constituents.

1. THE GRANITE-LIP AKITE GROUP

This group includes the most acid of all eruptive rocks; that
is, those which on analysis are found to yield the highest per-
centages of silica. Their chief essential constituents are quartz
and potash feldspars, while the more basic ferruginous minerals
are in quantities proportionately small. The group includes a
deep-seated or plutonic type, granite, and two effusive or vol-
canic types, quartz porphyry, and liparite or rhyolite. They
may be described in detail as below:

(1) THE GRANITES

Mineral Composition. The essential constituents of granite
are quartz and a potash feldspar (either orthoclase or micro-
cline), and plagioclase. Nearly always one or more minerals of
the mica, hornblende, or pyroxene group are present, and in
small, usually microscopic forms, the accessories magnetite,
apatite, and zircon; more rarely occur sphene, beryl, topaz,
tourmaline, garnet, epidote, allanite, fluorite, and pyrite. De-
lesse 1 has made the following determination of the relative pro-
portion of the various constituents in two well-known granites:

MINERAL COMPOSITION OF GRANITE

EGYPTIAN RED GRANITE

PARTS

PORPHYRITIO GRANITB, VOSGES

PARTS

R6d orthoclas6

W^hite orthoclase

White albite ....

Reddish oligoclase ....

Gray quartz

Black mica .

Mica

Total

100

Total

100

Perkins 2 gives the mineral composition of the "Medium Stock"
gray granite of Barre, Vermont, as follows :

1 Prestwich, Chemical and Physical Geology, Vol. I, p. 42.
2 Report State Geologist of Vermont, 1901-2.

BOCKS FORMED THROUGH IGNEOUS AGENCIES

Microcline .... 56.8 Biotite 10.2

Orthoclase 2.1 Muscovite 0.1

Plagioclase .... 1.3 Titanite 0.6

Quartz 28.5 Total 99.6

Chemical Composition. A general idea of the varying char-
acter of the granites may be gained from the f ollowing analyses :

CHEMICAL COMPOSITION OF GRANITE

KINDS AND LOCALITIES

Si0 2

A1 2 3

FeO
Fe 2 8

CaO

MgO

K 2

NajO

Biotite granite, near Dublin,
Ireland

73.0

13.64

2.44

1 84

2.11

4.21

3.53

Biotite granite, Silesia . .
Hornblende granite, Salt
Lake Utah ....

73.13

71.78

12.49
14.75

2.58
1.94 1

2.40
2.36

0.27
0.71

4.13
4.89

2.61
3.12

Gneissoid biotite granite,
District of Columbia . .
Hornblende mica granite,
Syene Egypt

69.33
68 18

14.33

16 20

3.60
4 10

3.21
1 75

2.44

2.67
6 48

2.70

2 88

Although the mineral apatite is so universally a constituent
of granitic rocks, yet it occurs in such small quantities as to
be quite overlooked in the ordinary methods of analysis. Such
tests as have been made show that the amount of phosphoric
acid (P 2 5 ) contained by rocks of this class rarely exceeds
0.2% and may fall as low as 0.05%. Small as is the amount,
it is nevertheless probable that it was from just such minute
quantities in granites and the more basic eruptives, that was
derived the main supply of phosphates existing in soils.

Structure. The granites are holocrystalline granular rocks.
As a rule none of the essential constituents show good crystal
outlines, though the feldspathic minerals are often quite perfectly
formed. The quartz has always been the last mineral to so-
lidify, and hence occurs only as irregular granules occupying the
interspaces. It is remarkable from its carrying innumerable
cavities filled with liquid and gaseous carbonic acid or with
saline matter. So minute are these cavities that it has been esti-
mated by Sorby that from one to ten thousand millions could
be contained in a single cubic inch of space. The microscopic

Yielded also 1.09% manganese oxide.

THE GRANITE-LIPAKITE GROUP

structure of a mica granite from Maine is shown in Fig. 3
and in Fig. 1, PI. 5.

The granites vary in texture almost indefinitely, presenting all
gradations from fine evenly granular rocks to coarsely porphy-
ritic forms in which the
feldspars, which are the
only constituents porphy-
ritically developed, are
several inches in length.

Colors. The prevail-
ing color is some shade of
gray, though greenish,
yellowish, pink, to deep
red, are not uncommon.
The various hues are due
to the color of the prevail-
ing feldspar and the
abundance and kind of
the accessory minerals.
Granites in which mus-
covite is the prevailing
mica, are nearly always
very light gray in color. The dark gray colors are due largely to
abundant black mica or hornblende, the greenish and pink or
red to the prevailing greenish, pink, or red feldspars.

Classification and Nomenclature. Several varieties are com-
monly recognized and designated by names dependent upon the
predominating accessory mineral. We thus have (1) musco-
vite granite, (2) biotite granite or granitite, (3) biotite-muscovite
granite, (4) hornblende granite, (5) hornblende-biotite granite,
and more rarely (6) pyroxene, (7) tourmaline and (8) epidote
granite. The name protogine, not now very generally used, has
been given to a granite in which the mica is in part or wholly
replaced by talc.

Graphic granite, or pegmatite, is a granitic rock consisting
essentially of quartz and orthoclase so crystallized together in
long parallel columns or shells that a cross-section bears a crude
resemblance to Hebrew writing. Aplit is a name used by the
Germans for a granite very poor in mica and consisting essen-
tially of quartz and feldspar only.

The names granitell and binary granite have also been used

FIG. 3. Microstructure of muscovite-bio-
tite granite, Hallowell, Maine.

64: BOCKS FOEMED THROUGH IGNEOUS AGENCIES

to designate rocks of this class. Greisen is a name applied to
a quartz-mica rock, with accessory topaz, occurring associated
with the tin ores of Saxony and regarded as a granite meta-
morphosed by exhalations of fluoric acid. Luxullianite and
Trowlesworthite are local names given to tourmaline or tour-
maline-fluorite granitic rocks occurring at Luxullian and
Trowlesworth, in Cornwall, England. The name Unakite has
been given to an epidotic granite with pink feldspars occurring
in the Unaka Mountains, western North Carolina.

The name granite porphyry is made to include a class of rocks
placed by Professor Rosenbusch under the head of dike rocks,
and differing from the true granites mainly in structural fea-
tures. They consist in their typical forms of orthoclase feldspars
and quartzes porphyritieally developed in a finer holocrystalline
aggregate of the minerals common to the granite group.

Geological Age and Mode of Occurrence. The granites are
massive rocks, occurring most frequently associated with the
older and lower rocks of the earth's crust, sometimes inter-
stratified with metamorphic rocks or forming the central por-
tions of mountain chains. They are not, as once supposed, the
oldest of rocks, but occur in eruptive masses or bosses invading
rocks of all ages up to late Mesozoic or Tertiary times. Profes-
sor Whitney considered the eruptive granites of the Sierra
Nevada to be Jurassic. Zirkel divided the granites described in
the reports of the 40th Parallel Survey into three groups: (1)
Those of Jurassic age; (2) those of Palaeozoic age, and (3) those
of Archa3an age. The granites of the eastern United States, on
the other hand, have, in times past, been regarded as mainly
Archaean, though Dr. Wadsworth has shown that the Quincy,
Massachusetts, stone is an eruptive rock of late Primordial or
more recent age, while Professor Hitchcock regards the eruptive
granites of Vermont as having been protruded during Silurian
or perhaps Devonian times.

The granites are among the most wide-spread and commonest
of igneous rocks, and are of great economic importance for
structural and monumental work. In the United States they are
to be found mainly in the Appalachian region and from the front
range of the Rocky Mountains westward to the Pacific coast.

THE QUARTZ PORPHYRIES 65

(2) THE QUARTZ PORPHYRIES

Composition. The mineral and chemical composition of the
quartz porphyries is essentially the same as that of the gran-
ites, from which they differ mainly in structure. Their essen-
tial constituents are quartz and feldspar, with accessory black
mica or hornblende in very small quantities; other acces-
sories present, as a rule only in microscopic quantities, are
magnetite, pyrite, hematite, and epidote.

Structure. The prevailing structure is porphyritic. (Fig. 1,
PL 2.) To the unaided eye they present a very dense and com-
pact ground-mass of reddish, brown, black, gray, or yellowish
color, through which are scattered clear glassy crystals of quartz
alone, or of quartz and feldspar together. The quartz differs
from that of the granites in having been the first mineral to
separate out on cooling it has taken on a more perfect crystalline
form; the crystal outlines of the feldspar are also well defined.
Under the microscope the ground-mass in the typical porphyry
is found to consist of a dense felsitic, almost irresolvable sub-
stance, which chemical analysis shows to be also a mixture of
quartzose and feldspathic material. The porphyritic quartzes
show frequently results of marked corrosion from the molten
magma, the mineral having again been partially dissolved after
its first crystallization. (Fig. 3, PI. 5.) This difference in
structure in rocks of the same chemical composition is believed
to be due wholly to the different circumstances under which
solidification has taken place. The structure of the ground-
mass is not always felsitic, but may vary from a glass through
spherulitic, micropegmatitic, and porphyritic to perfectly micro-
crystalline forms as in the microgranites. This difference in
structure may be best understood by reference to Plate 5, which
shows the microscopic structure of (1) granite from Sullivan,
Hancock County, Maine, (2) micropegmatite from Mount Desert,
Maine, and (3) a quartz porphyry from Fairfield, Pennsylvania.
Marked fluidal structure is common. (See PI. 2, Fig. 2.)

Colors. The colors of the ground-mass, as above noted, vary
through reddish, brownish gray to black and sometimes yellowish
or green. The porphyritic feldspars vary from red, pink, and
yellow to snow-white, and often present a beautiful contrast with
the ground-mass, forming a desirable stone for ornamental pur-

66 ROCKS FORMED THROUGH IGNEOUS AGENCIES

Classification and Nomenclature. Owing to the very slight
development of the accessory minerals, mica, hornblende, etc.,
it has been found impossible to adopt the system of classifica-
tion and nomenclature used with the granites and other rocks.
Vogelsang's classification as modified by Rosenbusch is based
upon the structure of the ground-mass as revealed by the micro-
scope. It is as follows:

Ground-mass holocrystalline granular Micro-granite.

Ground-mass holocrystalline, but formed of quartz and feld-
spar aggregates, rather than distinct crystals Granophyr.

Ground-mass felsitic Felsophyr.

Ground -mass glassy Vitrophyr.

Intermediate forms are designated by a combination of the
names, as granofelsophyr, felsovitrophyr, etc. The name felsite
is often given to members of this group in which the porphyritic
constituents are wholly lacking. The names felstone and petro-
silex were once common, though now out of use. Elvanite is a
Cornish miner's term and too indefinite to be of great value.
Eurite, now little used, was applied to felsitic forms. The
name felsite pitchstone or retinite has been given to a glassy
form with pitch-like lustre, such as occurs in dikes cutting the
old red sandstone on the Isle of Arran. Kugel porphyry is a
name given by German writers to varieties showing spheroids
with a radiating or concentric structure. Micropegmatite is the
term not infrequently applied to such as show under the micro-
scope a pegmatitic structure. (Fig. 2, PL 5.) Various popular
names, as leopardite and loadstone, are sometimes applied to such
as show a spotted or spherulitic structure.

(3) THE LIPARITES

Mineral Composition. These rocks may be regarded as the
younger equivalents of the quartz porphyries, or the volcanic
equivalents of the granites, having essentially the same mineral
and chemical composition. The prevailing feldspar is the clear
glassy variety of orthoclase known as sanidin ; quartz occurs in
quite perfect crystal forms often more or less corroded by the
molten magmas, as in the porphyries, and in the minute, six-
sided, thin platy forms known as tridymite. The accessory
minerals are the same as those of the granites and quartz
porphyries.

THE LIPAEITES

Chemical Composition. Below is given the composition of:
(I) Nevadite, from the northeastern part of Chalk Mountain,
Colorado, as given by Cross. 1 (II) That of a rhyolite form,
from the Montezuma Range, Nevada, as given by King, 2 and
(III) that of a black obsidian from the Yellowstone National
Park, Wyoming, as given by Iddings. 3

CHEMICAL COMPOSITION OF LIPARITE

CONSTITUENTS

III

Silica (Si0 2 )

74.50 %

74 62 L

74 70 V

Alumina (A1 2 O 3 )
Ferric oxide (Fe 2 C*3) ... ....

14.72
None

11.96
1.20

I-X.IU IQ

13.72
1 01

Ferrous oxide (FeO)

0.56

0.10

0.62

Ferric sulphide (FeS 2 )
Manganese (MnO)

0.28

0.40
Trace

Lime (CaO) .

0.83

0.36

Magnesia (MgO) .

0.37

0.14

Soda (Na 2 0)

3.97

2.26

3.90

Potash (K 2 0)

4.53

7.76

4.02

Phosphoric anhydride (P 2 Os)

0.01

Ignition

0.66

1.02

0.62

Specific gravity

100.38%

99.28 %
2 2

99.91 %
2 3447

Colors. These are fully as variable as in the quartz por-
phyries; white, though all shades of gray, green, brown, yel-
low, pink and red are common. Black is the more common
color for the glassy varieties of obsidian, though they are often
beautifully spotted and streaked with red or reddish-brown.

Structure. The liparites present a great variety of structural
features, varying from holocrystalline, through porphyritic and
felsitic, to clear, glassy forms. These varieties can be best
understood by reference to Plates 5 and 6, prepared from
photographs. A pronounced flow structure is quite character-
istic of the rocks of this group, as indicated by the name rhyolite.
The microscopic structure of an obsidian is shown in Fig. 4, PL

1 Geology and Mining Industry of Leadville, Monograph XII, U. S. Geol.
Survey, p. 349.

'Geological Exploration 40th Parallel, Vol. I, p. 652.
"Ann. Eep. U. S. Geol. Survey, 1885-86, p. 282.

68 EOCKS FOEMED THEOUGH IGNEOUS AGENCIES

5. Transitions from compact obsidian into pumiceous forms,
due to expansion of included moisture, are common.

Classification and Nomenclature. The following varieties
are now generally recognized, the distinctions being based mainly
on structural features, as with the quartz porphyries. We thus
have the granitic-appearing variety nevadite, the less markedly
granular and porphyritic variety rhyolite, and the glassy forms
hyaloliparite, hyaline rhyolite, or obsidian as it is variously
called. Hydrous varieties of the glassy rock with a dull pitch-
like lustre are sometimes called rhyolite pitchstone.

The name rhyolite, from the Greek word pew, to flow, it may
be stated, was applied by Richtofen as early as 1860 to this
class of rocks as occurring on the southern slopes of the Carpa-
thians. Subsequently Both applied the name Liparite to similar
rocks occurring on the Lipari Islands. The first name, owing
to its priority, is the more generally used for the group, though
Professor Rosenbusch in his latest work has adopted the latter.
The name Nevadite is from the state of Nevada, and was also
proposed by Richtofen. The name Obsidian as applied to the
glassy variety is stated to have been given in honor of Obsid-
ius, its discoverer, who brought fragments of the rock from
Ethiopia to Rome. The name pant client e has been given by
Rosenbusch to a liparite in which the porphyritic constituent
is anorthoclase.

Rocks of these types occur, in the United States, only in
the regions west of the front range of the Rocky Mountains.
Apo-rhyolite is the name proposed by Dr. Williams for the
devitrified and otherwise altered pre-Cambrian rhyolite found
at South Mountain in Pennsylvania.

2. THE SYENITE-TRACHYTE GROUP

This group stands next to that of the granites in point of
acidity, from which it differs mainly in the lack of free silica
(quartz) as an essential constituent. On chemical grounds this
and the next group to be described belong to the intermediate
series, standing midway between the acid granites and the basic
basalts. As with the last, there are plutonic and effusive forms.
These may be described as below:

THE SYENITES 69

(1) THE SYENITES

The name Syenite, from Syene, a town of Egypt. The word
was first used by Pliny to designate the coarse red granite from
quarries at Syene, used by the Egyptians in their obelisks
and pyramids. Afterwards (in 1787) Werner introduced the
word into geological nomenclature to designate a class of gran-
ular rocks consisting of feldspar and hornblende, either with or
without quartz. Later, when a more precise classification be-
came necessary, the German geologists reserved the name syenite
to designate only the quartzless varieties, while the quartz-
bearing varieties were referred to the hornblendic granites.
This is the classification now followed by the leading petrologists
and is therefore adopted here. Much confusion has arisen from
the fact that the French geologist Roziere insisted upon desig-
nating the quartz-bearing rock as syenite, a practice which has
been followed to a considerable extent both in this country and
England.

Mineral Composition. The syenites differ from the granites
only in the absence of the mineral quartz, consisting essentially
of orthoclase feldspar in company with biotite, or one or more
minerals of the amphibole or pyroxene group. A soda-lime
feldspar is nearly always present and frequently microcline;
other common accessories are apatite, zircon, and the iron ores:
more rarely sodalite.

Chemical Composition. In column I on p. 70 is given the
composition of a hornblende syenite from near Dresden, Saxony,
in II that of a mica syenite (minette) from Odenwald, in III
that of an augite-sodalite syenite from Montana, and in IV that
of an augite syenite from Franklin Co., New York.

Structure. The structure of the syenites is wholly analo-
gous to that of the granites, and need not be further described
here.

Color. The prevailing colors are various shades of gray,
through pink to reddish.

Classification and Nomenclature. According as one or the
other of the accessory minerals of the bisilicate group predomi-
nates we have (1) hornblende syenite, (2) mica syenite, or min-
ette, and (3) augite syenite.

Other varietal names have from time to time been given
by various authors. The name minette, first introduced into

70 ROCKS FOEMED THEOUGH IGNEOUS AGENCIES

CHEMICAL COMPOSITION OF SYENITE

CONSTITUENTS

III

Silica (Si0 2 )

60 02 L

57 37 /

54 15 /

6345%

Alumina (A^Og) ... .

16.66

13.84

18 92

18.31

Ferric iron (Fe 2 03)
Ferrous iron (FeO)

} 7.21

f 2.44
t 3 44

} 6.79

f 0.42
\ 356

Magnesia (MgO)

2 51

6 05

1 90

0.35

Lime (CaO) . .

3.59

5.53

3 72

2.93

Soda (Na 2 0)

2.41

1.53

5.47

5.06

Potash (K 2 0)

6.50

4.47

8.44

5.19

Ignition (H 2 0)

1.10

3.17

0.30

Chlorine (Cl.)

0.42

100.00%

97.84%

99.81

99.57%

geological nomenclature by Voltz in 1828 (Teall), is applied
to a fine-grained mica orthoclase rock, occurring only in the
form of dikes and further differing from the typical syenites in
having a porphyritic rather than granitic structure. Vogesite
is the name applied to a similar rock in which hornblende or
augite prevails in place of mica. These rocks are placed by
Professor Rosenbusch in his latest work in the group of syenitic
lamprophyrs. Monzonite is a varietal name for the augite syenite
of Monzoni in the Tyrol.

The mode of occurrence of the syenites is similar to that
of the granites, though they are much more limited in their
distribution. In the United States they have thus far been
described but sparingly. Marblehead Neck, Massachusetts;
Jackson, New Hampshire, are well-known localities; a beauti-
ful hornblende syenite is found among the glacial drift boulders
about Portland, Maine, but its exact source is not known. The
hornblende syenite described by Hawes as occurring at Red
Hill, Moultonborough, New Hampshire, has been shown by
Professor W. S. Bayley 1 to carry elaeolite, and to belong to the
group of elaeolite syenites. Hornblende syenites occur in the
Vosges Mountains of Germany and in Saxony; mica syenites
or minettes in the Odenwald, Germany, Baden, Saxony, and in
the Fichtelgebirge. A mica-augite syenite carrying sodalite
occurs as a Cretaceous eruptive in Jefferson County, Montana, 2

^ull. Geol. Soc. of America, Vol. Ill, 1892.

2 Proc. U. S. Nat. Museum, Vol. XVII, 1894

THE ORTHOCLASE OR QUARTZ-FREE PORPHYRIES 71

and a similar rock has been described by Lindgren from the
Highwood Mountains in the same state. 1

(2) THE ORTHOCLASE OR QUARTZ-FREE PORPHYRIES

Mineral Composition. The essential constituents are the
same as those of syenite. They consist therefore of a compact
porphyry groundmass with porphyritic feldspar (orthoclase)
and accessory plagioclase, quartz, mica, hornblende, or minerals
of the pyroxene group. More rarely occur zircon, apatite,
magnetite, etc., as in the syenites.

Chemical Composition. Being poor in quartz, these rocks are
a trifle more basic than the quartz porphyries which they other-
wise resemble. The following is the composition of an ortho-
clase porphyry from Pedazzo as given by Kalkowski; 2 Silica,
64.45% ; alumina, 16.31% ; ferrous oxide, 6.49% ; magnesia,
0.30% ; lime, 1.10% ; soda, 5.00% ; potash, 5.45% ; water, 0.85%.

Structure. Excepting that orthoclase is the porphyritic con-
stituent, they are structurally identical with the quartz porphy-
ries, and need not be further described here.

Colors. These are the same as the quartz porphyries already
described.

Classification and Nomenclature. The orthoclase or quartz-
free porphyries bear the same relation to the syenites as do the
quartz porphyries to granite, and the rocks are frequently
designated as syenite porphyries. Like the quartz porphyries,
they occur in intrusive sheets, dikes, and lava flows associated
with the Palaeozoic formations. Owing to the frequent absence
of accessory minerals of the ferro-magnesia group, the rocks can-
not in all cases be classified as are the syenites, and distinctive
names based upon other features are often applied. The term
orthophyr is applied to the normal orthoclase porphyries, and
these are subdivided when possible into biotite, hornblende, or
augite orthophyr according as either one of these minerals is the
predominating accessory. The term rhombporphyry has been
used to designate an orthoclase porphyry found in southern
Norway, in which the porphyritic constituent appears in char-
acteristic rhombic outlines, and which is further distinguished
by a complete absence of quartz and rarity of hornblende. The

1 Proc. Call. Acad. of Sciences, Vol. Ill, 2d series, p. 47.

2 Elemente der Lithologie, p. 86.

EOCKS FORMED THKOUGH IGNEOUS AGENCIES

name kerafophyr was given by Gumbel to a quartzose or quartz-
free porphyry containing a sodium-rich alkaline feldspar. So
far as can be at present judged, rocks of this type are much
more restricted in their occurrence than are the quartz porphyries
already described.

(3) THE TEACHYTES

Trachyte, from the Greek word r/oaxw, rough, in allusion to
the characteristic roughness of the rock. The term was first
used by Haiiy to designate the well-known volcanic rocks of the
Drachenfels on the Rhine.

Mineral Composition. Under the name of trachyte are com-
prehended those massive Tertiary and post-Tertiary lavas, con-
sisting essentially of sanidin with hornblende augite or black
mica, and which may be regarded as the younger equivalents of
the quartz-free porphyries. The common accessory minerals
are plagioclase, tridymite, apatite, sphene, and magnetite, more
rarely, sodalite, hauyne, and mellilite.

Chemical Composition. The following analyses show the
range in chemical composition of these rocks, I being that of
a trachyte from Game Ridge, Colorado, and II that of one from
San Pietro island, Sardinia.

CHEMICAL COMPOSITION OF TRACHYTE

CONSTITUENTS

Silica (SiO 2 ) ...

66.03%

56 09 /

Alumina (A^Os)

18.49

26 09

Ferric oxide (Fe2Os)

2.18

Manganese oxide (MnO)

Trace

Lime (CaO)

3 41

Maoiiesia (MgO) . . . . .

0.39

2 70

Potash (K 2 O) ....

6.86

6.49

Soda (Na 2 0)

6.22

3 38

Ignition (H 2 O)

0.85

1.05

Phosphoric acid (P20s)

0.04

Total

100.24 %

100.74 %

Structure. In structure the trachytes are rarely granular,
but possess a fine, scaly or microfelsitic ground-mass, rendered
porphyritic through the development of scattering crystals of

THE NEPHELINE SYENITES 73

sanidin, hornblende, augite of black mica. The texture is
porous, and the rock possesses a characteristic roughness to
the touch; hence the derivation of the name as given above.
Perlitic structure is common in the glassy forms. The micro-
scopic structure of the trachyte of Monte Vetta is shown in
Fig. 5, PL 5.

Colors. The prevailing colors are grayish, yellowish, or
reddish.

Classification and Nomenclature. They are divided into
hornblende, augite, or mica trachytes, according as any one of
these minerals predominates. The name sanidin-oligoclase trachyte
is sometimes given to trachytes in which both these feldspars ap-
pear as prominent constituents. The presence of quartz gives
rise to the variety quartz trachytes. (See under rhyolite.) The
glassy form of trachyte is commonly known under the name of
trachyte pitchsione, or if with a perlitic structure simply as per-
lite. In his most recent work Professor Rosenbusch has included
the glassy forms under the name of hyalotrachyte.

3. THE FOYAITE-PHONOLITE GROUP

This group differs from the last mainly in the partial replace-
ment of the potash feldspars by the closely related mineral
elaeolite or nepheline. In includes therefore those plutonic and
effusive rocks commonly known under the name of elceolite or
nepheline syenites and the phonolites. In their silica and potash
percentages it will be observed they differ not greatly from
the syenites proper, but are much more rich in soda and corre-
spondingly poor in lime. They may be described in detail as
follows :

(1) THE NEPHELINE (ELAEOLITE) SYENITES: FOYAITS

Nepheline from the Greek vc</>eAr;, a cloud, since the mineral
becomes cloudy on immersion in acid. Ekeolite from eAatov, oil,
in allusion to the greasy lustre. Syenite from Syene in Egypt.

Mineral Composition. The essential constituents of this
group are nepheline (elaeolite) and orthoclase, with nearly
always a pyroxenic or amphibolic mineral and a plagioclase
feldspar. The common accessory minerals are sphene, sodalite,
cancrinite, zircon, apatite, black mica, ilmenite and magnetite,
with occasional leucite, melinophane, and also tourmalines and

74 KQCKS FOKMED THEOUGH IGNEOUS AGENCIES

perowskite. Calcite, epidote, chlorite, analcite, and sundry min-
erals of the zeolite group occur as secondary products.

Professor W. S. Bayley has computed 1 the relative propor-
tions of the various constituents in the elaeolite syenite of Litch-
field, Maine, as follows: Elaeolite, 17%; potash feldspar, 27%;
albite, 47% ; cancrinite, 2% ; and black mica (lepidomelane), 7%.

Chemical Composition. The composition of the nepheline
syenite from several well-known localities is given below :

CHEMICAL COMPOSITION OF NEPHELINE SYENITE

CONSTITUENTS

ALGRAVE,
PORTUGAL

HOT SPRINGS,

ARKANSAS

LlTCHFIELD,

MAINE

BEEMERVILLE,
NEW JERSEY

Silica (SiO a )
Alumina (A^Os) ....
Ferric oxide (Fe 2 3 ) . . .
Ferrous oxide (FeO) . . .
Magnesia (MgO)
Manganese oxide (MnO) .
Liime (CaO)

54.61 %
22.07
2.33
2.50
0.88

2 51

59.70%
18.85
4.85

0.68
1 34

60.39%
22.51
.42
2.26
0.13
0.08
32

50.36%
19.84

J6.94

0.411
3 43

Soda (Na 2 0)

7 58

6 29

8 44

7 64

Potash (K 2 O) . ...

5.46

5.97

4 77

7 17

Titanium oxide (Ti0 2 ) . .
Phosphoric anhydride (P 2 Os)
Water (HgO)

0.09
0.15
1 13

1 88

3 512 (loss)

99.31

99.56

99.89

99.303

The essential points to be noted are the larger percentages
of the alkalies over those yielded by syenites of the ordinary
type, or the granites.

Color. The colors are light to dark gray, and sometimes
reddish.

Structure. The syenites, like the granites, are massive holo-
crystalline granular rocks, and as a rule sufficiently coarse in
texture to allow a determination of their essential constituents
by the unaided eye. In the Litchfield (Maine) syenite the
elaeolite often occurs in crystals upwards of 5 centimetres in
length, and zircons 2 centimeters in length are not rare. Neither
of the essential constituents occur in the form of perfect crystals,
while the apatite, zircon, black mica, and pyroxenes often pre-

1 Bull. Geol. Soc. of America, Vol. Ill, 1892, p. 231.

THE DIORITE-ANDESITE GROUP 75

sent very perfect forms. The cancrinite occurs both as secondary
after the elseolite and as a primary constituent in the form of
long needle-like yellow crystals with a hexagonal outline. This
last form is especially characteristic of the Litchfield rock.

Classification and Nomenclature. Several varietal names
have been given to the rocks of this group as described by various
authors. Miascite was the name given by G. Rose to the sye-
nite occurring at Miask in the Urals; Ditroite to that occurring
at Ditro in Transylvania, and Foyaite, by Blum, to that from
Mount Foya, in the province of Algrave in Portugal. The
name zircon syenite, or Laurvikite, has been given to the variety
from Laurvig in southern Norway, which is rich in zircons.
Tinguaite is the name proposed for a varietal form from Serra
de Tingua, province of Rio Janeiro, Brazil.

American petrographers have not been at all delinquent in
the matter of names, and have added to an already over-burdened
nomenclature such terms as Litchfieldite, Ouachitite, Pulaskite,
and Fourchite to varieties from Litchfield, Maine, and the Hot
Springs region of Arkansas. Liebnerite is the name given to an
ekeolite syenite porphyry occurring in the Tyrol.

Rocks of this group, although wide-spread in their distribu-
tion, are nevertheless not abundant. The more important
localities thus far described have already been noted; there
remains to be mentioned the locality at Red Hill, Moulton-
borough, New Hampshire, the rock of which was first described
as an ordinary syenite, and that of Hastings County, Ontario.

(2) THE PHONOLITES

Phonolite, from the Greek word <f><*>vij, sound, and Atfo?, stone,
in allusion to the clear ringing or clinking sound which slabs
of the stone emit when struck with a hammer; formerly called
clinkstone for the same reason.

Mineral Composition. The phonolites consist essentially of
sanidin and nepheline or leucite, together with one or more
minerals of the augite-hornblende group, and generally hauyne
or nosean. The common accessories are plagioclase, apatite,
sphene, mica, and magnetite; more rarely occur tridymite,
melanite, zircon, and olivine. The rock undergoes ready altera-
tion, and calcite, chlorite, limonite, and various minerals of the
zeolite group occur as secondary products.

76 BOCKS FOEMED THEOUGH IGIsEOUS AGENCIES

Chemical Composition. The average of six analyses given
by Zirkel 1 is as follows: Silica, 58.01%; alumina, 20.03%; iron
oxides, 6.18% ; manganese oxide, 0.58% ; lime, 1.89% ; magnesia,
0.80%; potash, 6.18%; soda, 6.35%; water, 1.88%; specific
gravity, 2.58.

Structure. The phonolites present but little variety in
structure, being usually porphyritic, seldom evenly granular.
The porphyritic structure is due to the development of large
crystals of sanidin, nepheline, leucite, or hauyne, and more rarely
hornblende, augite, or sphene, in the fine-grained and compact
ground-mass, which is usually microcrystalline, rarely glassy or
amorphous.

Colors. The prevailing colors are dark gray or greenish.

Classification and Nomenclature. Three varieties are recog-
nized, the distinction being founded upon the variation in pro-
portional amounts of the minerals, sanidin, nepheline, or leucite.
We thus have (1) nepheline phonolite, consisting essentially
of nepheline and sanidin, and which may therefore be regarded
as the volcanic equivalent of the nepheline syenite; (2) leucite
phonolite, consisting essentially of leucite and sanidin; and (3)
leucitophyr, which consists essentially of both nepheline and
leucite in connection with sanidin, and nearly always melanite.

So far as now known, these rocks are of comparatively rare
occurrence in the United States. The Black Hills of South
Dakota and the Cripple Creek district of Colorado are well-
known localities.

4. THE DIORITE-ANDESITE GROUP

The rocks of this and the succeeding group differ in a marked
degree from those discussed in previous pages, a difference due
in large part to an absence of orthoclase or other potash minerals
as an essential constituent. The group includes the plutonic
type diorite, and the effusive types hornblende porphyrite, and
andesite. These may be described as below:

(1) THE DIOEITES (GREENSTONES IN PART)
Diorite, from the Greek word 8iop^~iv , to distinguish. A term

first used by the mineralogist Haiiy.

Mineral Composition. The essential constituents of diorite

are plagioclase feldspar, either labradorite or oligoclase, and

1 Lehrbuch der Petrographie, II, p. 193.

PLATE

FIG. 1. Orbicular diorite.

FIG. 2. Granite spheroid.

7TsAj

*r THI

DIVERSITY

THE DIORITE-ANDESITE GROUP

hornblende or black mica. The common accessories are mag-
netite, titanic iron, orthoclase, apatite, quartz, augite, black mica,
and pyrite, more rarely garnets. Calcite, chlorite and epidote
occur as alteration products.

Structure. Diorites are holocrystalline granular massive
rocks. The texture is, as a rule, fine, compact, and homogeneous,
and its true nature discernible only with the aid of a microscope ;
more rarely porphyritic forms occur as in the camptonites. The
individual crystals are sometimes grouped in globular aggregates,
thus forming the so-called orbicular diorite, kugel diorite, or
napoleonite from Corsica. (Fig. 1, PL 7.)

Colors. The colors vary from green and dark gray to almost
black.

Chemical Composition. The following table shows the wide
range in chemical composition found in rocks commonly grouped
under this head.

CHEMICAL COMPOSITION or DIORITE

CONSTITUENTS

Silica (SiO 2 )

67.54%

61.75%

56.71%

50.47%

43.50%

Alumina (AloCX)

17.02

18 88

18 36 .

18 73

17 02

Ferric iron (Fe 2 O 3 )

2.97

0.52

4.19

13.68

Ferrous iron (FeO)

004

3 52

645

4 92

Lime (CaO)

2.94

3.54

6.11

8.82

8.15

Maemesia (MsrCO .

1.51

1.90

3.92

3.48

6.84

Potash (K 2 O)

2.28

1.24

2.38

3.56

2.84

Soda (Na 2 O)

4.62

3.67

3.52

4.62

2.84

Phosphoric acid (P 2 5 ) ....
Carbonic acid (CO 2 )

[ 0.55

4.46

0.58

Water (H 2 O)

4.35

I. Quartz-mica diorite: Electric Peak, Yellowstone Park (J. P. Iddings).
II.' Diorite: Penmaen-Mawr, Wales (J. A. Phillips). III. Diorite: Corn-
stock Lode, Nevada (40th Parallel Survey). IV, Augite diorite: Custer
County, Colorado (Whitman Cross). V. Porphyritic diorite (camptonite) :
Fairhaven, Vermont (J. F. Kemp).

Classification. Accordingly as they vary in mineral compo-
sition the diorites are classified as (1) diorite, in which horn-
blende alone is the predominating accessory; (2) mica diorite,
in which black mica replaces the hornblende, and (3) augite
diorite, in which the hornblende is partially replaced by augite.

78 EOCKS FOEMED THEOUGH IGNEOUS AGENCIES

The presence of quartz gives rise to the varieties, quartz, quartz-
augite, and quartz-mica diorites. The name tonalite was given
by Vom Rath to a quartz diorite containing the feldspar
andesine and very rich in black mica. Kersantite is a dioritic
rock consisting essentially of black mica and plagioclase, with
accessory apatite and augite, or more rarely hornblende, quartz,
and orthoclase. Professor Rosenbusch has placed the kersantites,
together with the porphyritic diorites (camptonites), under the
head of diorite lamprophyrs in the class of dike rocks. The
name, it should be stated, is from Kersanton, a small hamlet in
the Brest Roads, department of Finistere, France.

The diorites were formerly, before their exact mineral ogical
nature was well understood, included with the diabases and
melaphyrs under the general name greenstone. They are rocks
of wide geographic distribution, but apparently less abundant
in the United States than are the diabases. The lamprophyr
varieties are still less abundant, so far as now known.

(2) THE POEPHYEITES

Mineral and Chemical Composition. The essential constitu-
ents of the porphyrites are the same as of the diorites, from
which they differ mainly in structure.

Structure. The porphyrites, as a rule, show a felsitic or
glassy ground-mass, in which are embedded quite perfectly
developed porphyritic plagioclases, with or without hornblende
or black mica. At times, as in the well-known "porfido rosso
antico," or antique porphyries of Egypt, the ground-mass is
microcrystalline, forming thus connecting links between the true
diorites and diorite porphyrites. Indeed, the rocks of the group
may be said to bear the same relation to the diorites in the
plagioclase series as do the quartz porphyries to the granites in
the orthoclase series, or better yet, they may be compared with
the hornblende andesites, of which they are apparently the
Palaeozoic equivalents.

Colors. The prevailing colors are dark brown, gray, or
greenish.

Classification. According to the character of prevailing
accessory mineral, we have hornblende porphyrite, or diorite
porphyrite, as it is sometimes called, and mica porphyrite.
When neither of the above minerals are developed in recognizable

THE PORPHYRTTES AND ANDESTTES

quantities, the rock is designated as simply porphyrite. The
porphy rites are wide-spread rocks, very characteristic of the
later Palaeozoic formations, occurring as contemporaneous lava
flows, intrusive sheets, dikes, and bosses.

(3) THE ANDESITES

The name Andesite was first used by L. Von Buch in 1835, to
designate a type of volcanic rocks found in the Andes Moun-
tains, South America.

Mineral Composition. The essential constituents are soda-
lime feldspar, together with black mica, hornblende, augite, or
a rhombic pyroxene, and in smaller, usually microscopic pro-
portions, magnetite, ilmenite, hematite, and apatite. Common
accessories are olivine, sphene, garnets, quartz, tridymite, anor-
thite, sanidin, and pyrite.

Chemical Composition. The composition of the andesites
varies very considerably, the quartz-bearing members naturally
showing much the higher percentage of silica. The following
table shows the composition of a few typical forms :

CHEMICAL COMPOSITION OF ANDESITE

CONSTITUENTS

III

Silica (Si0 2 )

66.32 %

69.51%

61.12 %

56.07 %

56.19%

5833 /

Alumina (A1 2 O 8 ) . . .
Ferric oxide (Fe 2 O 3 ) . .
Ferrous oxide (FeO) .
Magnesia (MgO) . . .
Lime (CaO) .

14.33
5.53
0.25
2.45
4 64

15.75
3.34

2.09
1 71

11.61
11.64

0.61
4 33

19.06
5.39
0.92
2.12
7 70

16.21
4.92
4.43
.4.60
7 00

18.17

6.03
2.40
6 19

Soda (Na 2 O) . . .

3.90

3 89

3.85

4 52

2 96

320

Potash (K 2 0) ....
Water (H 2 0) ....

1.61
1.13

3.34

3.52
4.35

1.24
0.99

2.37
1.03

3.02
0.76

100.16%

99.63 %

101.03%

98.01 %

99.62

98.10%

I. Dacite from Kis Sebes, Transylvania. II. Dacite from Lassens Peak,
California. III. Hornblende andesite from hill north of Gold Peak,
Nevada. IV. Hornblende andesite from Bogoslof Island, Alaska. V. Hy-
persthene andesite, Buffalo Peaks, Colorado. VI. Augite andesite from
north of American Flat, Washoe, Nevada.

Structure. To the unaided eye the andesites present as a
rule a compact, often rough and porous ground-mass carrying

80 ROCKS FORMED THROUGH IGNEOUS AGENCIES

porphyritic feldspars and small scales of mica, hornblende, or
whatever may be the prevailing accessory; pumiceous forms
are not uncommon. Under the microscope the ground-mass is
found to vary from clear glassy through microlitic forms to
almost holocrystalline. The minerals of the ground-mass are
feldspars in elongated microlites, specks of iron ore, apatite in
very perfect forms, and one or more of the accessory ferro-mag-
nesian minerals.

Colors. The prevailing colors are some shade of gray, green-
ish or reddish.

Classification and Nomenclature. Specific names are given
dependent upon the character of the prevailing accessory. We
thus have: Quartz andesites or dacites; Hornblende andesites;
Augite andesites; Hypersthene andesites; and Mica andesites.

The glassy varieties are often known as hyaline andesites.
The name propylite was given by Richtofen to a group of
andesitic rocks prevalent in Hungary, Transylvania, and the
western United States but the rocks have since been shown by
Dr. Wadsworth 1 and others to be but altered andesites, and the
name has fallen largely into disuse.

5. THE GABBRO-BASALT GROUP

This is a large and variable group of rocks which on struc-
tural and mineralogical grounds might well be subdivided. Thus
the gabbros, norites, and hypersthene andesites might well be
considered as a group by themselves, while the diabases, augite
porphyrites, melaphyrs, and basalts would form a second. Ow-
ing, however, to the similarity of the magmas from which they
have been derived, it is believed the wants of the student will be
best subserved by grouping them all together as above. They
may be described in detail as below :

(1) THE GABBROS

Gabbro, an old Italian name originally applied to serpen-
tinous rocks containing diallage.

Mineral Composition. The gabbros consist essentially of a
basic soda-lime feldspar, either labradorite, bytownite, or an-
orthite, and diallage or a closely related monoclinic pyroxene,
a rhombic pyroxene (enstatite or hypersthene), and more rarely

1 Proc. Boston Society of Natural History, Vol. XXI, 1881, p. 260.

THE PORPHYRITES AND ANDESITES

olivine. Apatite and the iron ores are almost universally pres-
ent, and often picotite, chromite, pyrrhotite, more rarely com-
mon pyrites, and a green spinel. Secondary brown mica and
hornblende are common. Quartz occurs but rarely.

Chemical Composition. As with other groups, the percent-
age amounts of the various constituents obtained by analyses
is dependent upon the relative proportion of the constituent
minerals. In the tables given below, analyses like I and III,
showing very little iron and magnesia, but rich in lime and
soda and alumina, are of rocks in which the pyroxenic con-
stituents are almost wholly lacking, and which consist essen-
tially of lime feldspars only.

CHEMICAL COMPOSITION OP GABBRO

CONSTITUENTS

III

Silica (SiO 2 ) ....
Alumina (A1 2 3 ) . . .
Ferric iron (Fe 2 3 ) . .
Ferrous iron (FeO) . .
Lime (CaO)

59.55 %
25.62
0.76

7.73

54.72 %
17.79
2.08
6.03
6.84

53.43%
28.01
0.75

11.24

49.15%
21.90
6.60
4.54
8.22

46.85%
19.72
3.22
7.99
13.10

45.66 %
16.44
0.66
13.90
7.23

Magnesia (MgO) . . .
Potash (K 2 0) ....
Soda (Na 2 O)

Trace
0.96
5 09

5.85
3.01
3.02

0.63
0.96

4.85

3.03
1.61
3.83

7.75
0.09
1.56

11.57
0.41
2.13

Ignition and loss . . .

0.45

1.92

0.56

0.07

100.15$

99.34$

99.83$

99.80$

100.84$

98.07$

I. Anorthosite: Chateau Richer, Canada (T. S. Hunt). II. Gabbro: near
Cornell Dam, Croton River, New York (J. F. Kemp). III. Anorthosite:
Labrador (A. Wickman). IV. Gabbro: near Duluth, Minnesota (Streng).
V. Gabbro: near Baltimore, Maryland (G. H. Williams). VI. Gabbro:
Northwest Minnesota (W. S. Bay ley).

Structure. The gabbro structure is quite variable. Like
the other plutonic rocks mentioned, they are crystalline granu-
lar, the essential constituents rarely showing perfect crystal
outlines. As a rule the pyroxenic constituent occurs in broad
and very irregularly outlined plates, filling the interstices of
the feldspars, which are themselves in short and stout forms
quite at variance with the elongated, lath-shaped forms seen in
diabases. This rule is, however, in some cases reversed, and
7

82 EOCKS FOBMED THEOUGH IGNEOUS AGENCIES

the feldspars occur in broad, irregular forms surrounding the
more perfectly formed pyroxenes. Transitions into diabase struc-
ture are not uncommon. Through a molecular change of the
pyroxenic constituent, the gabbros pass into diorites, as do also
the diabases.

Colors. The prevailing colors are gray to nearly black;
sometimes greenish through decomposition.

Classification. The rocks of this group are divided into (1)
the true gabbros that is, plagioclase-diallage rocks and (2)
norites, or plagioclase-bronzite and hypersthene rocks. Both
varieties are further subdivided according to the presence or
absence of olivine. We then have :

True gabbro = Plagioclase -\- diallage.
Olivine gabbro Plagioclase -}- diallage and olivine.
Norite = Plagioclase -|- hypersthene or bronzite.

Olivine norite = Plagioclase + hypersthene and olivine.

Nearly all gabbros contain more or less rhombic pyroxene, and
hence pass by gradual transitions into the norites. Through
a diminution in the proportion of feldspar they pass into the
peridotites, and a like diminution in the proportion of pyroxene
gives rise to the so-called forellenstein. Hyperite is the name
given, by Tornebohm, to a rock intermediate between normal
gabbro and norite. Anorthosite is the name given to the granular
varieties poor or quite lacking in pyroxenes.

(2) THE DIABASES

Diabase, from the Greek word Sia/3a<ns, a passing over; so
called by Brongni'art because the rock passes by insensible grada-
tions into diorite.

Chemical Composition. The table on page 83 shows the aver-
age range in composition of (I and II) the plutonic diabase and
(III, IV, V, and VI) the effusive forms melaphyr and basalt.

Mineral Composition. The essential constituents of diabase
are plagioclase feldspar and augite, with nearly always mag-
netite and apatite in microscopic proportions. The common
accessories are hornblende, black mica, olivine, enstatite, hyper-
sthene, orthoclase, quartz, and titanic iron. Calcite, chlorite,
hornblende, and serpentine are common as products of altera-
tion. Through a molecular change known as uralitization the

THE DIABASES 83

CHEMICAL COMPOSITION OF DIABASE AND BASALT

CONSTITUENTS.

III

Silica (SiO 2 ) . . .

53.13f c

45.46%

56.52$

51.02%

57.25%

46.90%

Alumina (A1 2 O 3 ) .

13.74

19.94

13.53

18.36

16.45

10.17

Ferric iron (Fe 2 O 3 ) .
Ferrous iron (FeO) .

1.08
i 9.10

} 15.36

12.56

/6.57
14.68

1.67
4.72

1.22

5.17

Lime (CaO) . . .

9.47

8.32

5.31

7.36

7.65

6.20

Magnesia (MgO)

8.58

2.95

2.79

5.57

6.74

20.98

Potash (K 2 O). . .

1.03

3.21

3.59

2.10

1.57

2.04

Soda.(Na 2 O) . . .

2.30

2.12

3.71

2.54

3.00

1.16

Ignition ....

0.90

2.30

0.81

2.86

0.40

5.42

99.33%

99.66%

98.82%

101.56%

100.35%

99.26%

Specific gravity . .

2.96

2.945

....

2.86

I. Diabase: Jersey City, New Jersey (G. W. Hawes). II. Diabase: Pal-
mer Hill, Au Sable Forks, New York (J. F. Kemp). III. Melaphyr:
Hockenberg, Silesia. IV. Melaphyr, Falgendorf, Bohemia (quoted from
Zirkel's Lehrbuch der Petrographie). V. Quartz basalt: Snag Lake, Cali-
fornia (J. S. Diller). VI. Basalt (absarokite) : near Bozeman, Montana
(G. P. Merrill).

augite may become converted into hornblende, as already de-
scribed (p. 36), and the rock pass over into diorite. The plagio-

clase may be oligoclase,
labradorite, or anorthite.
Structure. In struc-
ture the diabases are holo-
crystalline. Rarely do
the constituents possess
perfect crystal outlines,
but are more or less im-
perfect and distorted,
owing to mutual inter-
ference in process of for-
mation, the granular
hypidiomorphic structure
of Professor Rosenbusch.
The augite in the typical
forms occurs in broad and
sharply angular plates en-
closing the elongated or lath-shaped crystal of plagioclase, giving

FIG. 4. Microstructure of diabase.

84 KOCKS FOEMED THROUGH IGNEOUS AGENCIES

rise to a structure known as ophitic. ( See Fig. 4. ) The rocks are
compact, fine, and homogeneous, though sometimes porphyritic
and more rarely amygdaloidal.

Colors. The colors are sombre, varying from greenish
through dark gray to nearly black, the green color being due to
a disseminated chloritic or serpentinous product resulting from
the alteration of the augite or olivine.

Classification. Two principal varieties are recognized, the
distinction being based upon the presence or absence of the
mineral olivine. We thus have: (1) diabase proper and (2) oli-
vine diabase.

Many varietal names have been given from time to time by
different authors. Gumbel gave the name of leucophyr to a
very chloritic, diabase-like rock consisting of pale green augite
and a saussurite-like plagioclase. The same authority gave
the name epidorite to an altered diabase rock occurring in small
dikes in the Fichtelgebirge, and in which the augite had become
changed to hornblende. He also designated by the term pro-
terobase a Silurian diabase consisting of a green or brown,
somewhat fibrous hornblende, reddish augite, two varieties of
plagioclase, chlorite, ilmenite, a little magnetite, and usually a
magnesian mica. The name ophite was used by Pallarson to
designate an augite plagioclase eruptive rock, rich in horn-
blende and epidote, occurring in the Pyrenees. The researches
of M. Levy Kuhn 1 and others showed, however, that both horn-
blende and epidote are secondary, resulting from the augitic
alteration, and that the rock must be regarded as belonging to
the diabases.

The Swedish geologist, Tornebohm, gave the name saklite dia-
base to a class of diabasic rocks containing the pyroxene sahlite,
which occurred in the province of Smaaland, and in other
localities. The name teschenite was for many years applied to
a class of rocks occurring in Moravia, which, until the recent
researches of Kohrbach, were supposed to contain nepheline, but
which are now regarded as merely varietal forms of diabase.
Variolite is a compact, often spherulitic, variety occurring in
some instances as marginal facies of ordinary diabase. The
name eukrite or eucrite was first used by G. Rose to designate
a rock consisting of white anorthite and grayish green augite

1 Untersuchungen iiber pyrenaeische Ophite, Inaugural Dissertation Uni-
versitat, Leipzig, 1881.

THE MELAPHYRS AND AUGITE PORPHYBITES 85

occurring in the form of a dike in the Carlingford district,
Ireland.

The diabases are among the most abundant and wide-spread
of the so-called trap rocks, occurring in the form of dikes, in-
trusive sheets, and bosses. They are especially characteristic
of the Triassic formations of the eastern United States. It
should be noted, however, that many of these Triassic traps
have been shown to be true lava flows, and that on both litho-
logical and geological grounds such may be classed with the
basalts.

(3) THE MELAPHYRS AND AUGITE PORPHYRITES

The term melaphyr is used to designate a volcanic rock
occurring in the form of intrusive sheets and lava flows, and
consisting essentially of a plagioclase feldspar, augite, and
olivine, with free iron oxides and an amorphous of porphyry
base. The augite porphyrites differ in containing no olivine.
The rocks of this group are therefore the porphyritic, effusive
forms of the olivine-bearing and olivine-free diabases and
gabbros.

Structure. As above noted, they are porphyritic rocks with,
in their typical forms, an amorphous base, are often amygda-
loidal, and with a marked flow structure.

Colors. In colors they vary through gray or brown to nearly
black; often greenish through chloritic and epidotic decompo-
sition.

Classification and Nomenclature. According as olivine is
present or absent, they are divided primarily into melaphyrs
and augite porphyrites, the first bearing the same relation to
the olivine diabases as do the quartz porphyries to the granites,
or the hornblende porphyrites to the diorites, and the second
a .similar relation to the olivine-free diabases. The augite
porphyrites are further divided upon structural grounds into
(1) diabase porpJiyrite, which includes the varieties with holo-
crystalline diabase granular ground-mass of augite, iron ores,
and feldspars, in which are embedded porphyritic lime-soda
feldspars, mainly labradorite, idiomorphic augites, and at
times accessory hornblende and black mica; (2) spilite, which
includes the non-porphyritic compact, sometimes amygdaloidal
and decomposed forms such as are known to German petrog-
raphers as dichte diabase, diabase mandelstein (amygdaloid),

86 KOCKS FOKMED THEOUGH IGNEOUS AGENCIES

kalk-diabase, variolite, etc.; (3) the true augite porphyrite, in-
cluding the normal porphyritic forms with the amorphous base,
and (4) the glassy variety augite vitrophyrite.

(4) THE BASALTS

Basalt, a very old term used by Pliny and Strabo to designate
certain black rocks from Egypt which were employed in the arts
in early times. 1

Mineral Composition. The essential minerals are augite and
plagioclase feldspar with olivine in the normal forms ; accessory
magnetite and ilmenite, together with apatite, are always pres-
ent, and more rarely a rhombic pyroxene, hornblende, black mica,
quartz, perowskite, hauyne and nepheline, and minerals of the
spinel group. Metallic iron has been found as a constituent of
certain basaltic rocks of Greenland.

Chemical Composition. The composition is quite variable,
as shown by analyses in columns V and VI on p. 83. The fol-
lowing shows the common extremes of variation: Silica, 45%
to 55% ; alumina, 10% to 18% ; lime, 7% to 14% ; magnesia,
3% to 10% ; oxide of iron and manganese, 9% to 16% ; potash,
0.058% to 1.50% ; soda, 2% to 5% ; loss by ignition, 1% to 5% ;
specific gravity, 2.85 to 3.10.

Structure. Basalts vary from clear glassy to holocrystalline
forms. The common type is a compact and, to the unaided eye,
homogeneous rock, with a splintery or conchoidal fracture, show-
ing only porphyritic olivines in such size as to be recognizable.
Under the microscope they show a ground-mass of small feldspar
and augite microlites, with perhaps a sprinkling of porphyritic
forms of feldspar, augite, and olivine, and a varying amount of
interstitial brownish glass. Pumiceous and amygdaloidal forms
are common.

Colors. The prevailing colors are dark, some shade of gray
to perfectly black. Red and brown colors are also common.

Classification and Nomenclature. In classifying, the varia-
tions in crystalline structure are the controlling factors. As,
however, these characteristics are such as may vary almost
indefinitely in different portions of the same flow, the rule has
not been rigidly adhered to here. We thus have :

(1) Dolerite, including the coarse-grained almost holocrys-

1 Teall, British Petrography, p. 136.

THE BASALTS 87

talline variety; (2) anamesite, including the very compact fine-
grained variety, the various constituents of which are not dis-
tinguishable by the unaided eye; (3) basalt proper, which in-
cludes the compact homogeneous, often porphyritic, variety,
carrying a larger proportion of interstitial glass or devitrifica-
tion products than either of the above varieties, and (4) tachy-
lite, hyalomelan, or hyalobasalt, which includes the vitreous or
glassy varieties, the mass having cooled too rapidly to allow it to
assume a crystalline structure. These varieties, therefore, bear
the same relation to normal basalt as do the obsidians to the
liparites. Other varieties, though less common, are recogniz-
able and characterized by the presence or absence of some pre-
dominating accessory mineral. We have thus quartz, hornblende,
and hypersthene basalt, etc. An olivine-free variety is also
recognized.

The basalts are among the most abundant and wide-spread of
the younger eruptive rocks. In the United States they are
found mainly in the regions west of the Mississippi River.
They are eminently volcanic rocks, and occur in the form of lava
streams and sheets, often of great extent, and sometimes show-
ing a characteristic columnar structure. According to Rich-
thofen, the basalts are the latest products of volcanic activity.
A quartz-bearing basalt has been described by Mr. J. S. Diller
as occurring at Snag Lake, near Lassens Peak, California, 1 which
is regarded by him as a product of the latest volcanic eruption
within the limits of the state.

Under the name of melilite basalt is included a group of racks
in which the mineral melilite is the characterizing constituent,
with accessory augite, olivine, nepheline, biotite, magnetite,
perowskite, and spinel. The normal structure is holocrystal-
line porphyritic, in which the olivine, augite, mica, or occasion-
ally the melilite, appear as porphyritic constituents. These are
rocks of very limited distribution, and at present known in
North America only near Montreal, Canada, Little Falls, N. Y.,
and Southern Texas. Professor Rosenbusch, in his later work,
separates this entirely from the basalts, and considers it in a
group by itself under the name of Melilite Rocks.

1 Bull. No. 79, U. S. Geol. Survey, 1891.

88 BOOKS FOKMED THEOUGH IGNEOUS AGENCIES

6. THE THERALITE-BASANITE GROUP

This is a small, and so far as now known, comparatively in-
significant group of rocks, representatives of which are confined
to limited and widely separated areas. They are described as
below :

(1) THE THERALITES

The name theralite, derived from the Greek word %>av, to seek
eagerly, was given by Professor Rosenbusch to a class of intru-
sive rocks consisting essentially of plagioclase feldspar and
nepheline, and which are apparently the plutonic equivalents of
the tephrites and basanites.

The group was founded upon certain rocks occurring in dikes
and laccoliths in the Cretaceous sandstones of the Crazy Moun-
tains of Montana, and described by Professor J. E. Wolff, 1 of
Harvard University.

Mineral Composition. The essential constituents as above
noted are nepheline and plagioclase with accessory augite, olivine,
sodalite, biotite, magnetite, apatite and secondary hornblende
and zeolitic minerals.

Chemical Composition. The chemical composition of a sam-
ple from near Martinsdale, as given by Professor Wolff, is as
follows: Silica, 43.175%; alumina, 15.236%; ferrous oxide,
7.607%; ferric oxide, 2.668%; lime, 10.633%; magnesia,
5.810% ; potash, 4.070% ; soda, 5.68% ; water, 3.571% ; sulphuric
anhydride, 0.94%.

Structure. The rocks are holocrystalline granular through-
out.

Colors. These are dark gray to nearly black.

The theralifos, so far as known, have an extremely limited
distribution, and in the United States have thus far been re-
ported only from Gordon's Butte and Upper Shields River Basin
in the Crazy Mountains of Montana.

(2) THE TEPHEITES AND BASANITES

Mineral Composition. The essential constituent of the rocks
of this group as given by Rosenbusch are a lime-soda feldspar
and nepheline or leucite, either alone or accompanied by augite.
Olivine is essential in basanite. Apatite, the iron ores, and
rarely zircon occur in both varieties. Common accessories are

1 Notes on the Petrography of the Crazy Mountains and other localities in
Montana, by J. E. Wolff. Neues Jahrb. fur Min., 1885, I, p. 69 ; 1890, I, p.
192.

THE THERALITE-BASANITE GROUP

sanidin, hornblende, biotite, hauyne, melanite, perowskite, and
a mineral of the spinel group.

Chemical Composition. The following is the composition of

(I) a nepheline tephrite from Antao, Pico da Cruz, Azores, and

(II) a nepheline basanite from San Antonio, Cape Verde
Islands, as given by Roth. 1

CHEMICAL COMPOSITION OF TEPHRITE AND BASANITE

CONSTITUENTS

Silica (SiO 2 )

47 44 %

43 09 %

Alumina (A1 2 8 )

23.71

1745

Iron sesquioxide (F'^Os)

683

1899

Iron protoxide (FeO)

353

Magnesia (MgO)

1 95

463

Lime (CaO)

6.47

976

Soda (Na 2 O)

6.40

5.02

Potash (K2O)

334

1 81

Water (H 2 O)

1.73

033

101.40%

101.08%

Structure. The rocks of this group are as a rule porphyritic
with a holocrystalline ground-mass, though sometimes there is
present a small amount of amorphous interstitial matter or
base; at times amygdaloidal.

Colors. The colors are dark, some shade of gray or brownish.

Classification and Nomenclature. According to their vary-
ing mineral composition Rosenbusch divides them into: Leucite
tephrite, Leucite basanite, Nepheline tephrite, Nepheline ba-
sanite.

The group, it will be observed, stands intermediate between
the true basalts and the nephelinites to be noted later. Their
distribution, so far as now known, is quite limited.

7. THE PERIDOTITE-LIMBURGITE GROUP

This and the following groups include eruptive rocks in
which neither quartz nor feldspars of any kind longer appear
as essential constituents. They are therefore very low in silica,
and classed as ultrabasic. Although in most cases comparatively
insignificant as rock masses, they are peculiarly interesting as

1 Abhandlungen der Konig. Akad. der Wissenschaften zu Berlin, 1884,
p. 64.

EOCKS FOEMED THEOUGH IGNEOUS AGENCIES

mineral aggregates, and even more on account of the character
of their alteration products. The peridotites are further of
interest in presenting the nearest homologues to meteorites of
any of our terrestrial rocks. The group includes the plutonic
peridotites (serpentine in part), and effusive picrite porphy rites
and limburgites. In detail these are as below:

(1) THE PEEIDOTITES

Peridotite, so called because the mineral peridot (olivine) is
the chief constituent.

Mineral Composition. The essential constituent is olivine
associated nearly always with chromite or picotite and the iron
ores. The common accessories are one or more of the ferro-
magnesian silicate minerals augite, hornblende, enstatite, and
black mica ; and more rarely feldspar, apatite, garnet, sillimanite,
perowskite, and pyrite.

Chemical Composition. The chemical composition varies
somewhat with the character and abundance of the prevailing
accessory. The following table shows the composition of several
typical varieties.

CHEMICAL COMPOSITION OF THE PERIDOTITES

CONSTITUENTS.

III

Silica (SiO 2 ) ....

41.58%

43.84%

39.103%

42.94%

38.01%

45.68%

Alumina (A1 2 O 3 ) . .

0.14

1.14

4.94

10.87

5.32

6.28

Magnesia (MgO) . .

49.28

44.33

29.176

16.32

23.29

34.76

Lime (CaO) ....

0.11

1.71

3.951

9.07

4.11

2.15

Iron sesquioxide (Fe 2 O 3 )

4.315

3.47

6.70

9.12

Iron protoxide (FeO) .

7.49

8.76

11.441

10.14

4.92

Chrome oxide (Cr 2 O 3 ) .

....

0.42

0.436

0.26

Manganese (MnO) . .

....

0.12

0.276

Trace

Potash (K 2 O) ....

....

Trace

0.15

0.22

....

Soda(Na 2 O) . . . .

....

0.90

4.15

....

Nickel oxide (NiO) . .

0.34

0.51

....

Water and ignition . .

1.72

1.06

5.669

6.09

10.60

1.21

100.66%

101.89%

99.307%

99.95%

97.32%

99.46%

Specific gravity . . .

....

3.287

2.93

2.88

2.83

3.269

I. Dunite: Macon County, North Carolina. II. Saxonite: St. Paul's
Eocks, Atlantic Ocean. III. Picrite: Nassau, Germany. IV. Hornblende
picrite: Ty Cross, Anglesia. V. Picrite: Little Deer Isle, Maine. VI.
Lherzolite: Monte Eossi, Piedmont.

THE PERIDOT1TES

Structure. The structure as displayed in the different varie-
ties is somewhat variable. In the dunite it is as a rule even
crystalline granular, none of the olivines showing perfect crystal
outlines. In the picrites the augite or hornblende often occurs in
the form of broad plates
occupying the interstices
of the olivines and wholly
or partially enclosing
them, as in the hornblende
picrite of Stony Point,
New York. The saxonites
and Iherzolites often show
a marked porphyritic
structure produced by
the development of large
pyroxene crystals in the
fine and evenly granular
ground-mass of olivines.
(See Fig. 5, as drawn

by Dr. G. H. Wil- FlG 5._Microstructure of porphyritic Iherzo-
liams.) The rocks belong ii te , partly altered into serpentine,
to the class designated as

hypidiomorphic granular by Professor Rosenbusch; that is,
rocks composed only in part of minerals showing crystal faces
peculiar to their species.

Colors. The prevailing colors are green, greenish gray, yel-
lowish green, dark green to black.

Nomenclature and Classification. Mineralogically and geo-
logically it will be observed the peridotites bear a close resem-
blance to the olivine diabases and gabbros, from which they
differ only in the absence of feldspars. Indeed, Professor Judd
has shown that the gabbros and diabase both, in places, pass by
insensible gradations into peridotites through a gradual dimi-
nution in the amount of their feldspathic constituents. Dr.
Wadsworth would extend the term peridotite to include rocks
of the same composition, but of meteoric as well as terrestrial
origin, the condition of the included iron, whether metallic or
as an oxide, being considered by him as non-essential, since
native iron is also found occasionally in terrestrial rocks, as
the Greenland basalts and some diabases.

92 BOOKS FOKMED THROUGH IGNEOUS AGENCIES

In classifying the peridotites the varietal distinctions are
based upon the prevailing accessory mineral. We thus have :

Dunite, consisting essentially of olivine only.

Saxonite, consisting essentially of olivine and enstatite.

Picrite, consisting essentially of olivine and augite.

Hornblende picrite, consisting essentially of olivine and hornblende.

Wehrlite (or eulysite), consisting essentially of olivine and diallage.

Lherzolite, consisting essentially of olivine, enstatite and augite.

The name Dunite was first used by Hochstetter and applied
to the olivine rock of Mount Dun, New Zealand. Saxonite
was given by Wadsworth, rocks of this type being prevalent in
Saxony. The same rock has since been named Harzburgite by
Rosenbusch. The name Lherzolite is from Lake Lherz in the
Pyrenees.

The peridotites are, as a rule, highly altered rocks, the older
forms showing a more or less complete transformation of their
original constituents into a variety of secondary minerals. The
most common result of this alteration is the rock serpentine, the
transformation taking place through the hydration of the olivine
and the liberation of free iron oxides and chalcedony. ( See Fig. 5. )
The chemistry of the process has been already discussed under
the head of olivine, p. 23. The prevailing color is some shade of
green, though not infrequently brown, yellow, red, or nearly
black.

(2) THE PICRITE PORPHYRITES

Under this head is placed a small group of rocks so far as
now known very limited in their distribution, which are regarded
as the effusive forms of the plutonic picrites, as bearing the same
relation to these rocks as do the melaphyrs to the olivine diabases.
The essential constituents are therefore olivine and augite with
accessory apatite, iron ores, and other minerals mentioned as
occurring in the true picrites. Structurally they differ from
these rocks in presenting an amorphous base rather than being
crystalline throughout. The group is quite limited in the United
States. Elliott County, Kentucky; Pike County, Arkansas; and
Syracuse, Onondaga County, New York, are well-known occur-
rences.

THE PYROXENITE-AUGITITE GROUP 93

(3) THE LIMBURGITES

This is a small group of lavas described by Rosenbusch in
1872 as occurring at Limburg, on the Rhine. The essential con-
stituents are augite and olivine with the usual iron ores. Struc-
turally they are never holocrystalline, but glassy and porphyritic.
The composition of the Prussian limburgite is given as below.

CHEMICAL COMPOSITION OF LIMBURGITE

CONSTITUENTS

PKK CENT

Silica (Si0 2 )

42 24

Aluiiiiiui (A^Os) . ...

18 66

Iron sesquioxide (Fe^Qs)

7 45

Magnesia (MffO)

12 27

Lime (CaO)

11 76

Soda (Na20)

4 02

Potash (K20) .... ....

1 08

Water (H 2 0)

3 71

101.19

So far as known, the group has no representatives in the
United States.

8. THE PTTROXENITE-AUGITITE GROUP

Here are included a small group of eruptive rocks differing
from the last mainly in the absence of olivine as an essential
constituent. They are represented, so far as now known, only
by the plutonic pyroxenites and effusive augitites.

(1) THE PYROXENITES

Pyroxenite, a term applied by Dr. Hunt to certain rocks con-
sisting essentially of minerals of the pyroxene group, and which
occurred both intrusive and as beds or nests intercalated with
stratified rocks. The author here follows the nomenclature and
classification adopted by Dr. G. H. Williams. 1

Mineral Composition. The essential constituents are one or
more minerals of the pyroxene group, either orthorhombic or
monoclinic. Accessory minerals are not abundant and are
limited mainly to the iron ores and minerals of the hornblende
or mica groups.

1 American Geologist, Vol. VI, July, 1890, pp. 35-49.

EOCKS FORMED THROUGH IGNEOUS AGENCIES

Chemical Composition. The following analyses serve to show
the variations which are due mainly to the varying character of
the pyroxenic constituents:

CHEMICAL COMPOSITION OF THE PYROXENITES

CONSTITUENTS

III

Silica (Si0 2 )

50 80 %

53 98 L

55 14 / A

Alumina (A^Os) ...

3.40

1.32

0.66

Chrome oxide (Cr 2 08)

032

053

025

Ferric oxide (Fe 2 0s) .

1 39

1 41

348

Ferrous oxide (FeO) .

811

390

473

Manganese (MnO)
Lime (CaO)

0.17
12.31

0.21
1547

0.03
839

Magnesia (MgO)

22.77

22.59

26.66

Soda (Na 2 0)

Trace

030

Potash (K 2 0)

Trace

Water (H 2 0). . .

052

083

038

Chlorine (Cl) . . .

024

023

100.03 %

100.24 %

100.25 %

I. Hypersthene-diallage rock: Johnny Cake Road, Baltimore County,
Maryland. II. Hypersthene-diallage rock: Hebbville post-office, Baltimore
County, Maryland. III. Bronzite-diopside rock from near Webster, North
Carolina.

Structure. The py-
roxenites are holocrystal-
line granular rocks, at
times evenly granular and
saccharoidal, or again
porphyritic, as in the
websterite from North
Carolina. The micro-
scopic structure of this
rock is shown in Fig. 6
from the original draw-
ing by Dr. Williams.

Colors. The colors
are, as a rule, greenish or

FIG. 6. Microstructure of websterite, Web- br nze -

ster, North Carolina. Classification and No-

menclature. The pyrox-
enites, it will be observed, differ from the peridotites only in the

THE PYEOXENITE-AUGITITE GROUP

lack of olivine. Following Dr. William's nomenclature, we have
the varieties diallagite, bronzitite, and kyperstkenite, according
as the mineral diallage, bronzite, or hypersthene forms the essen-
tial constituent. Websterite is the name given to the bronzite-
diopside variety, occurring near Webster, North Carolina, and
komblendite to the hornblende-augite variety. The pyroxenites
rank, in geological importance, next to the peridotites. Through
processes of hydration and other chemical changes, they pass into
amphibolic and steatitic masses to which the name soapstone or
potstone is applied. These last are dark gray or greenish rocks,
soft enough to be readily cut with a knife and with a pronounced
soapy or greasy feeling; hence the name soapstone. The name
potstone was given on account of their having been utilized for
making rude pots, for which their softness and fireproof proper-
ties render them well qualified. Although it is commonly stated
in the text-books that soapstone is a compact form of steatite or
talc, few are even approximately pure forms of this mineral, but
all contain varying proportions of chlorite, mica, and tremolite,
together with perhaps unaltered residuals of pyroxene, granules
of iron ore, iron pyrites, quartz, and, in seams and veins, calcite
and magnesian carbonates. The variation in chemical composi-
tion is shown in the following analyses, I being that of a com-
pact, homogeneous-appearing, quite massive variety from Al-
berene, in Albemarle County, Virginia, and II one from Frances-
town, New Hampshire.

CHEMICAL COMPOSITION OF THE SOAPSTONES

CONSTITUENTS

Silica (Si02)

39.06%

42.43%

Alumina (A1 2 3 )

12.84

6.08

Ferric and ferrous iron (Fe 2 3 ) and (FeO) ....

12.90
5.98

13.07
3.27

22.76

25.71

Potash (K2O)

0.19

032

Soda (Na 2 0)

0.11

0.16

Ignition

6.56

8.45

100.40%

99.49%

(2) AUGITITE

The effusive form, augitite, differs from the pyroxenite proper
mainly on structural grounds. In common with many lavas it

BOCKS FOKMED THROUGH IGNEOUS AGENCIES

has a glassy base, in which are embedded the crystals of augite
and iron ores- The composition of an augitite from the Cape
Verde Islands, as given by Roth, is as below:

CHEMICAL COMPOSITION OF AUGITITE

CONSTITUENTS

PER CENT

Silica (SiO 2 )

41.83

Alumina (Al 2 0a)

18.60

Iron sesquioxide (Fe 2 0a)

16.11

Magnesia (MgO) ....

4 98

Lime (CaO) . .

11 83

Soda (Na 2 O)

4 70

Potash (K 2 0)

2.47

Water (H 2 O)

0.91

101.43

9. THE LEUCITE-NEPHELINE ROCKS

Under this head are grouped two small but interesting groups
of effusive rocks, having, so far as known, no exact equivalent
among the plutonics, and characterized by the presence of leu-
cite or nepheline, which here seem to play the role of feldspars
as essential constituents. In detail they are as below :

(1) THE LEUCITE BOCKS

Mineral Composition. The essential constituent is leucite
and augite. A variety of accessories occur, including biotite,
hornblende, iron ores, apatite, olivine, plagioclase, nepheline,
melilite, and more rarely garnets, hauyne, sphene, chromite, and
perowskite. Feldspar as an essential fails entirely.

Chemical Composition. The average chemical composition as
given by Blaas 1 is as follows: Silica, 48.9%; alumina, 19.5%;
iron oxides, 9.2% ; lime, 8.9% ; magnesia, 1.9% ; potash, 6.5% ;
soda, 4.4%.

Structure. The rocks of this group are, as a rule, fine
grained and only slightly vesicular, presenting to the unaided
eye little to distinguish them from the finer-grained varieties
of ordinary basalt.

Colors. The prevailing colors are some shades of gray,
though sometimes yellowish or brownish.

1 Katechismns cler Petrographie, p. 117.

THE NEPHELINE ROCKS

Classification and Nomenclature. The varietal distinctions
are based upon the presence or absence of the mineral olivine
and upon structural grounds and various minor characteristics.
We have the olivine-free variety leucitite and the olivine-holding
variety leucite basalt.

These rocks have also a very limited distribution, and, so far
as known, are found within the limits of the United States only
at the Leucite Hills, Wyoming, and the Highwood Mountains of
Montana.

(2) THE NEPHELINE EOCKS

Mineral Composition. These rocks consist essentially of
nepheline with augite and accessory sanidin, plagioclase, mica,
olivine, leucite, minerals of the sodalite group, magnetite, apa-
tite, perowskite, and melanite.

Chemical Composition. Below is given the composition of
(I) a nephelinite from the Cape Verde Islands, and (II) a
nepheline basalt from the Vogelsberg, Prussia. 1

CHEMICAL COMPOSITION OF NEPHELINE ROCKS

, . .
CONSTITUENTS

miica fSiOo'l

46.95 %

42.37 %

Alumina (A^Oa)

21.59

8.88

8.09

11.26

7.80

2.49

13.01

7.97

10.93

Soda fNaof)^

8.93

4.51

2.04

1.21

Water (H 2 0)

2.09

0.34

100. 15 #

100.29%
3.103

Colors. The prevailing colors are various shades of gray
to nearly black.

Structure. Structurally they are porphyritic, with a holo-
crystalline or in part amorphous base, usually fine grained and
compact, at times amygdaloidal.

Classification and Nomenclature. These rocks differ from
the basalts, which they otherwise greatly resemble, in that they

1 Roth, Abhandl. der Konig. Preus. Akad. der Wiss. zu Berlin, 1884.
8

98 BOOKS FORMED THROUGH IGNEOUS AGENCIES

bear the mineral nepheline in place of feldspar. Based upon the
presence or absence of olivine, we have, first, nepheline basalt,
and second, nephelinite. The name nepheline dolerite has been
given in some cases to the coarser, holocrystalline, olivine-bearing
varieties.

Like the leucite rocks, the members of this group are some-
what limited in their distribution.

II. AQUEOUS BOCKS

1. BOOKS FORMED THROUGH CHEMICAL AGENCIES

This comparatively small, though by no means unimportant,
group of rocks comprises those substances which, having once
been in a condition of aqueous solution, have been deposited as
rock masses either by cooling, evaporation, by a diminution of
pressure, or by direct chemical precipitation. It also includes
the simpler forms of those produced by chemical changes in
pre-existing rocks. Water, when pure or charged with more
or less acid or alkaline material, and particularly when acting
under great pressure, is an almost universal solvent. Thus,
heated alkaline waters, permeating the rocks of the earth's
crust at great depths below the surface, are enabled to dis-
solve from them various mineral matters with which they come
in contact. On coming to the surface or flowing into crevices,
the pressure is diminished, or evaporation takes place, and the
water, no longer able to carry its load, deposits it wholly or in
part as vein material or a surface coating. In other cases alka-
line or acid waters bearing mineral matters, may, in course of
their percolations, be brought in contact with neutralizing solu-
tions, and the dissolved materials be deposited by direct precipi-
tation. In these various ways were formed the rocks here de-
scribed. It will be observed that the various members of the
group are composed mainly of minerals of a single species only.

This group cannot be separated by any sharp lines from
that which is to follow, inasmuch as many rocks are not the
product of a single agency, acting alone, but are rather the
result of two or more combined processes. This is especially the
case with the limestones. It is safe to assume that few of these
are due wholly to accumulations of calcareous, organic remains,
but are, in part at least, chemical precipitates, as is well illus-
trated by the oolitic varieties.

According to their chemical nature, the group is divided
into (1) Oxides, (2) Carbonates, (3) Silicates, (4) Sulphates,
(5) Phosphates, and (6) Chlorides.

100 AQUEOUS EOCKS

(1) OXIDES

Here are included those rocks consisting essentially of oxygen
combined with a base, though usually other constituents are
present as impurities.

Hematite. Anhydrous sesquioxide of iron. Fe 2 3 = oxy-
gen, 30% ; iron, 70%. In nature nearly always more or less im-
pure through the mechanical admixture of argillaceous silicates
or calcareous matter, manganese oxides, sulphur, phosphates,
etc. Several forms are recognized, the distinction being based
mainly upon physical properties. Specular hematite is a mica-
ceous or foliated variety with a black, metallic, often splendent
lustre; this variety is mainly a metamorphic form, and prop-
erly should be classed with the metamorphic rocks. Compact,
columnar, fibrous, and earthy forms also occur, the latter often
known as ochre, as are similar forms of limonite. Although
classified here under the head of aqueous rocks, it does not
follow that the hematites have all originated in precisely the
same manner. To a limited extent the specular variety is found
about volcanic craters and fumaroles, where it was orginally
deposited by a process of sublimation. Through a process of
oxidation, beds of magnetic iron become locally altered into
hematite, giving rise to pseudomorphous granular, octahedral,
and dodecahedral forms, to which the name martite is given.
Many extensive beds undoubtedly arise from the dehydration
of dynamic agencies the folding and metamorphosing of the
enclosing rocks of beds of limonite. Others, like the fossil
and oolitic ores of the Clinton formations, arise in part from a
process of chemical precipitation and subsequent segregation,
the ore being originally disseminated throughout a ferruginous
limestone, and having accumulated as an insoluble residue as
the lime carbonate was carried away through the action of car-
bonated waters. The extensive hematite deposits of the Lake
Superior region of Michigan are regarded as oxidation prod-
ucts from pre-existing carbonates (siderite), the oxide having
been precipitated from solution in synclinal troughs, and subse-
quently crystallized by metamorphism. 1 The ores of the Mesabi
range, on the other hand, are regarded by at least one writer
as having originated through a somewhat complicated process
of oxidation and metasomatosis, whereby a ferruginous silicate
(greenalite) became converted into an admixture of free iron

1 Van Hise Monograph XIX, U. S. Geol. Survey, 1892.

PLATE 8

FIG. 1. Botrvoidal hematite.

FIG. 2. Clay-iron stone septarian nodule.

OXIDES 101

oxide and silica, the one or the other, according to the inter-
mittent character of the permeating solutions, being leached out
and redeposited at no great distance in a fair condition of
purity. 1 A discussion of this subject belongs more properly to
economic geology, and need not be dwelt upon further here.

Limonite (Brown Iron Ore). Iron sesquioxide plus water.
H 6 Fe 2 6 + Fe 2 3 . An earthy or compact dark brown, black,
or ochreous-yellow rock, containing, when pure, about two-
thirds its weight of metallic iron. It occurs in beds, veins, and
concretionary forms, associated with rocks of all ages, and
forms a valuable ore of iron. (See Fig. 1, PL 8.) On the bot-
toms of lakes, bogs, and marshes it often forms in extensive
deposits, where it is known as bog-iron ore. The formation of
these deposits is described as follows: Iron is widely diffused
in rocks of all ages, chiefly in the form of (1) the protoxide,
which is readily soluble in waters impregnated with carbonic
or other feeble acids, or (2) the peroxide, which is insoluble in
the same liquids. Water percolating through the soils becomes
impregnated with these acids from the decomposing organic
matter, and then dissolves the iron protoxide with which it
comes in contact. On coming to the surface and being exposed
to the air, as in a stagnant lake or marsh, this dissolved oxide
absorbs more oxygen, becoming converted into the insoluble
sesquioxide, which floats temporarily on the surface as an oil-
like, iridescent scum. Finally this sinks to the bottom, where
it gradually becomes aggregated as a massive iron ore. This same
ore may also form through the oxidation of pyrite, or beds of
ferrous carbonate. At the Ktaadn Iron Works, in Piscataquis
County, Maine, the ferrous salt, as it oxidizes, is deposited as a
coating over the leaves and twigs scattered about, forming thus
beautifully perfect casts, or fossils.

Pyrolusite, Psilomelane, and Wad. These are names given
to the anhydrous and more or less hydrated forms of manganese
oxides, which, though wide in their distribution, are found in
such abundance as to constitute rock masses in comparative
rarity. The origin of such deposits is at times somewhat ob-
scure. In all cases they are doubtless secondary. The original
source of the material appears to have been the manganiferous
silicates of Archaean and more recent eruptive rocks, whence it

*J. E. Spurr, Bull. No. 10, Geol. and Nat. Hist. Survey of Minnesota,
1894. Also C. K. Leith, Mono. 43, TJ. S. Geol. Survey.

102

AQUEOUS BOCKS

was derived by leaching, being transported in the form of
soluble salts and finally precipitated as oxide or carbonate, the
latter being subsequently converted into oxide. The deposits
occur as a rule in residual clays, as interbedded strata in shales
and sandstones, or as occupying superficial seams and joints,
and in the form of pockets and nests. True fissure veins of man-
ganese oxide are not known.

Beauxite (so called from Beaux, near Aries, France) is the
name given to a somewhat indefinite mixture of alumina and
iron oxides, occurring in the form of compact concretionary
grains of a dull red, brown, or nearly white color, and also
in compact and earthy forms. The mode of occurrence of the
mineral is somewhat variable. At Beaux and several other
localities it occurs in pockets in limestone, and also in beds
alternating with limestones, sandstones, and clays belonging
to the Cretaceous period. In the Puy-de-D6me the beds rest
directly upon gneiss, and are overlaid by basalt. At Oberhes-
sen, Germany, the mineral occurs in rounded masses embedded
in clay, as is also the case at Vogelsberg. In America, beaux-
ite has been found in Alabama, Georgia, and Arkansas. In
Alabama and Georgia it occurs in beds of irregular extent,
associated with limestones of Upper Cambrian age (the Knox
dolomite) ; in Arkansas the deposits are of Tertiary age.

The origin of the beauxite is somewhat obscure. It has been
argued that the beds at Beaux, and those of Var, are deposits
from mineral springs. Those of the Puy-de-D6me, the West-
erwald, Vogelsberg, and of Ireland, on the other hand, are
regarded as derived from basalt by a metasomatic process.

CHEMICAL COMPOSITION OF BEAUXITE

CONSTITUENTS

III

Silica (SiO^) .

2.8%

1 10 %

21.08 %

2.80%

10.38 %

Alumina (AlaOa) . .

57.6

50.92

48.92

52.21

55.64

Iron sesquioxide (Fe 2 O 3 ) .

25.3

15.70

2.14

13.50

1.95

Water (H 2 O)

10.08

27.75

23.41

27.72

27.62

Titanium oxide (Ti0 2 ) .

3.1

3.20

2.52

3.52

3.50

I. Beaux, France. II. Vogelsberg, Germany. III. Jacksonville, Alabama.
IV. Floyd County, Georgia. V. Pulaski County, Arkansas.

OXIDES 103

The Alabama and Georgia deposits, like those of Beaux, are
regarded as of chemical origin. 1

The material from various sources varies greatly in chemical
composition, as shown by the analyses on page 102.

Silica. Silica, as has been already noted under the head of
rock-forming minerals, is one of the most abundant constituents
of the earth's crust. In its various forms, which are sufficiently
extensive to constitute rock masses, it is always of chemical
origin, that is, results by deposition from solution, by precipi-
tation, or evaporation, as noted above. Varietal names are
given to the deposits, dependent upon their structure, method
of formation, color, and degree of purity. Siliceous sinter,
geyserite, and fiorite are names given to the nearly white,
often soft and friable, hydrated varieties formed on the evapo-
ration of the siliceous waters of hot springs and geysers, or
through the eliminating action of algous vegetation. The ma-
terial is, in reality, an impure form of opal. Throughout the
geyser regions of the Yellowstone Park, Iceland, and New
Zealand, the sinter has been deposited as a comparatively thin
crust over the surface, or in the form of cones about the throats
of the geysers. The varieties of silica known as opal are hydrous
forms occurring in veins and pockets, in a variety of rocks.
Frequently it forms the replacing material in silicified or "petri-
fied" woods. In the old lake beds of the Madison valley, Mon-
tana, are found large logs composed wholly of this material, no
sign of organic matter remaining, but yet with the woody struc-
ture beautifully preserved. Chalcedony is the translucent, mas-
sive, cryptocrystalline variety of silica occurring" mainly in
cavities in older rocks, where it has been deposited by infiltra-
tion. It is a common secondary product formed during the
decomposition of many rocks, and, like opal, may form the
petrifying medium of fossil woods and other organisms. Not
infrequently, also, it occurs in continuous layers of several inches
or even feet in thickness, interstratified with limestone. Flint
is a variety of chalcedony formed by segregation in chalky lime-
stone, and is composed, in part, of the broken and partially dis-
solved spicules of sponges, and the siliceous casts of infusoria.
Chert is an impure flint. It occurs in rounded, nodular, con-
cretionary masses interbedded with limestones, particularly

1 See resume of the subject, by E. L. Packard, in Mineral Eesources of
the United States for 1891.

104 AQUEOUS EOCKS

Palaeozoic varieties, and doubtless originated as did the flints
in the chalky limestones. Jasper is a dull or bright red, or yellow
variety of chalcedony containing alumina, and owing its color to
iron oxides.

The name novaculite is given to a very fine-grained and com-
pact chalcedony, such as is suitable for hones. As commonly
used, the name is made to include rocks of widely different
origin, some of which are evidently chemical precipitates, while
others are indurated clastic or schistose rocks. The well-known
novaculites of Arkansas are clear white masses of chalcedonic
silica, containing scattering quartz granules, minute grains of
garnet, and numerous small rhomboidal cavities which seem-
ingly were once occupied by crystals of calcite or dolomite.
Opinions differ as to the origin of this rock. Owen 1 regarded
it as a sandstone metamorphosed by percolating hot water.
Branner 2 looked upon it as a metamorphosed chert; Griswold, 3
as a sedimentary deposit in the form of siliceous silt on a sea-
bottom, while Rutley 4 argues that it is but a siliceous replace-
ment of beds of dolomite or dolomitic limestone. It seems
probable that the views of Branner or Rutley are the most
nearly correct.

Quartz is a massive form of crystalline silica occurring in
veins, disseminated granules, and pockets in rocks of all kinds
and all ages. It is one of the most wide-spread and commonest
of minerals, and is often of a pink or rose color from metallic
oxides. Lydian stone is an exceedingly hard impure quartz rock,
of a black color and splintery fracture. It was formerly much
used in testing the purity of precious metals.

(2) CARBONATES

Water carrying small amounts of carbonic acid readily dis-
solves the calcium carbonate of rocks with which it comes in
contact; on eyaporation and through loss of a portion of the
carbonic acid, this is again deposited. In this way are formed
numerous and at times extensive deposits, to which are given
varietal names dependent upon their structure and the special
conditions under which they originated. Calc sinter or tufa is

1 2d Rep. Geological Reconnaissance of Arkansis, 1860.

2 Ann. Rep. Geol. Survey of Arkansas, Vol. I, 1886, p. 49.
8 Ann. Rep. Geol. Survey of Arkansas, Vol. Ill, 1890.

4 Quarterly Journal Geological Society of London, August, 1894.

CARBONATES

105

a loose friable deposit made by springs and streams either by
evaporation or through intervention of algous vegetation. Such
are often beautifully arborescent and of a white color, as seen
at the Mammoth Hot Springs of the Yellowstone National Park.
Somewhat similar deposits are formed by springs in Virginia,
California, Mexico, New Zealand.

Tufa deposits of peculiar imitative shapes have been described
by Mr. I. C. Russell of the United States Geological Survey,
as formed by the evaporation of the waters of Pyramid Lake,
Nevada. Oolitic and piso-
litic limestones are so
called on account of their
rounded, fish-egg-like
structure, the word oolite
being from the Greek
work MOV, an egg. (See
PL 11.) These are in
part chemical and in part
mechanical deposits. The
water in the lakes and
seas in which they were
formed became so satur-
ated that the lime was
deposited in concentric

coatings about the grains ,, _ ,,.

FIG. 7. Microstructure of oolitic limestone,
of calcareous sand on

the bottom, and finally the little granules thus formed became
cemented into firm rock by the further deposition of lime
in the interstices. This structure will be best understood by
reference to Fig. 7. Rocks of this nature are now forming along
the beaches of Pyramid Lake.

Such forms as these may or may not show a nucleus. It
seems safe to assume that such a nucleus, at first, in all cases
existed, though it may be in microscopic dimensions only.

Travertine is a compact and usually crystalline deposit formed,
like the tufas, by waters of springs and streams. The traver-
tines are often beautifully veined and colored by metallic oxides
and form some of the finest marbles. Such are the so-called
"onyx marbles " of Mexico and Arizona. 1

1 The Onyx Marbles, Ann. Rep. U. S. National Museum for 1893. Also
Stones for Building and Decoration, Wiley & Sons, New York, 2d ed., p. 120.

106 AQUEOUS KOCKS

Stalactite and stalagmite are the names given to the deposits
formed from the roofs and on the floors of caves; water, perco-
lating through the limestone roof, by virtue of the carbonic acid
it contains, dissolves out a small amount of the lime, which, on
evaporation, is again deposited either as pendent cones from
the ceiling, or as massive and pillar-like forms upon the floor.
The pendants are known as stalactites; the corresponding
growths upon the floor as stalagmites. Stalactite and stalag-
mite sometimes meet, forming thus continuous pillars, or col-
umns extending from floor to ceiling. The lime of these
deposits, it may be said, is as a rule in the form of calcite,
though sometimes, as in the old portions of the Wyandotte
caves in Indiana, it is aragonite. The so-called " oriental ala-
baster" of the ancients is a stalagmitic deposit derived in part
from crevices and pockets in the Eocene limestones of the Nile
valley.

Magnesite, a carbonate of magnesia, occurs frequently as a
secondary mineral in the form of veins in serpentinous rocks,
but rarely itself forms rock masses of any importance. Rhodo-
chrosite, a carbonate of manganese, sometimes occurs in rock
masses, but is found most commonly in the form of veins asso-
ciated with ores of silver, lead, or copper.

Another carbonate, less common than that of lime, but which
sometimes occurs in such quantities as to constitute true rock
masses, is siderite, or carbonate of iron. A common form of
this is dull brownish or nearly black in color, very compact and
impure, containing varying amounts of calcareous, clayey, and
organic matter. In this condition it is found in stratified beds
and in the shape of rounded and oval nodules, or concretions,
which are called clay -ironstone nodules, septaria, and spkcero-
siderite. (See Fig. 2, PL 8.) These septarian nodules are
often beautifully veined with calcite, and when cut and polished
form not undesirable objects of ornamentation.

(3) SILICATES

Silica, combined with magnesia and water, gives rise to an
interesting group of serpentinous and talcose substances, which
are often sufficiently abundant to constitute rock masses. Pure
serpentine consists of about equal parts of silica and magnesia,
with from 12 to 13% of water. It is a compact, amorphous, or
colloidal rock, soft enough to be cut with a knife, with a slight

SILICATES

107

greasy feeling and lustre, and of a color varying from dull
greenish and almost black, through all shades of yellow, brown-
ish, and red. It also occurs in fibrous and silky forms, filling
narrow veins in the massive rocks, and is known as amianthus,
or chrysotile. It is very doubtful if serpentine is ever an original
rock; it is rather an alteration product of other and less stable
magnesian minerals. Here will be considered only those which
have originated by a series of chemical changes known as meta-
somatosis, a process of indefinite substitution and replacement,
in simple mineral aggregates occurring associated with the
older metamorphic rocks. Such are the serpentines derived
from non-aluminous pyroxenes, -like those of Montville, New
Jersey, and Moriah, New York, and those from Easton, Penn-
sylvania, derived from a massive tremolite rock. The analyses
given below will serve to illustrate the chemical changes which
occur in this process of metasomatosis, I being that of a nearly
white pyroxene, and II that of the serpentine derived therefrom.

ANALYSES SHOWING CHANGE OF PYROXENE TO SERPENTINE

CONSTITUENTS

Silica (Si0 2 )

64.215 %

42.38 %

Magnesia (MgO)

19.82

42.14

Lime (CaO)

24.71

0.00

0.59

0.07

Ferric oxide (Fe203)

0.20

Ferrous oride (FeO)

0.27

Ignition (HgO) . . ...

0.14

14.20

99.945 %

99.83 %

The pyroxene, it should be observed, occurs in nodular
masses in a crystalline granular dolomite. Various stages of
the process are shown in Fig. 8, in which the white and gray
central portions are nucleal masses of unchanged pyroxenes,
surrounded by the darker crusts of secondary serpentine. 1 Ser-
pentine as an alteration product of the mineral chondrodite is
also known to occur, though this form is less common. At Brew-
ster, New York, are extensive deposits of this nature. (See fur-
ther on p. 137.)

J See On the Serpentine of Montville, New Jersey, Proc. U. S. National
Museum, Vol. XI, 1888, p. 105. .

108 AQUEOUS EOCKS

Several varieties of serpentine are popularly recognized.
Precious or noble serpentine is simply a very pure compact va-
riety of a deep oil-yellow or green color. Amianthus, or chryso-
tile, as noted above, is the name given to the fibrous variety.
Williamsite is a deep bright green, trans-
lucent, and somewhat scaly variety, oc-
curring associated with the chrome iron
deposits in Fulton township, Lancaster
County, Pennsylvania. Deweylite is a
hard, translucent variety occurring in
veins in altered dunite beds. Bowenite
is a pale green variety forming veins in
limestone at Smithfield, Rhode Island.
Picrolite, marmolite, and retinolite are
varieties of minor importance. Serpen-
tine alone, or associated with calcite and

. dolomite, forms a beautiful marble, to
FIG. 8. Pyroxene partially . ' '

altered to serpentine. which the names verd antique, ophite,
and ophiolite are given. The so-called

Eozoon Canadense, a supposed fossil rhizopod, is a mixture
of serpentine and calcite or dolomite. The name serpentine is
from the latin serpentinus, serpent-like, in allusion to its green
color and often mottled appearance.

Those serpentines which were derived from basic eruptives,
or complex metamorphic rocks are described with those rocks
with which, in their unaltered state, they would naturally be
grouped.

The mineral steatite, or talc, when pure, differs from ser-
pentine in containing 63.5% of silica, 31.7% of magnesia, and
4.8% of water. Its common form is that of white or greenish
inelastic scales, forming an essential constituent of the talcose
schists. As is the case with serpentine, it sometimes results
from the alteration of eruptive magnesian rocks, such as the
pyroxenites, and rarely occurs as a direct result of precipitation.

Pyrophyllite, or agalmatolite, is a hydrous silicate of alumina,
somewhat harder than talc, which it otherwise resembles, and
which is used in making slate pencils and small images. It
occurs in a schistose form in the Deep River region of North
Carolina.

Kaolin, also a hydrous silicate of alumina, is a chemical
product in that it is a residue left by the chemical decomposi-

SULPHATES 109

tion of the feldspars. These minerals, as explained elsewhere,
consist of silicates of alumina and lime, with more or less of
the alkalies potash and soda, and iron oxides. In the process
of decomposition new compounds are formed, the more soluble
of which are leached out, leaving the less soluble silicates,
including kaolin, behind in a condition of more or less purity.
The material is of great value for fictile purposes, and is de-
scribed more fully under the head of argillaceous fragmental
rocks.

(4) SULPHATES

Gypsum. The rock gypsum is chemically a hydrous sul-
phate of lime, that is to say, consists of sulphur, lime, and
water, in the proportion of 32.6 parts of lime and 20.9 parts
of water, combined with 46.5 parts of sulphur trioxide. When
crystallized, the mineral is nearly colorless and transparent,
and splits readily into thin, inelastic sheets. The compact
massive varieties are white, gray to black, and sometimes pink
from various impurities. The most characteristic feature is
its softness, which is such that it can be readily cut with a
knife or even by the thumbnail.

Four varieties of gypsum are recognized: (1) The common
massive form, dull in color and often more or less impure;
(2) the pure white, fine-grained variety, alabaster; (3) the
fibrous variety, satin spar; and (4) the broadly foliated, trans-
parent variety, selenite, so called from the Greek word Getevt,
the moon, in allusion to its soft and pleasing lustre.

The following is an analysis of a commercial gypsum from
Ottawa County, Ohio, as given by Professor Orton i 1

Lime (CaO) 32.52%

Sulphuric acid (SO 3 ) .... 45.56

Water (H 2 O) 20.14

Magnesia (MgO) 0.56

Alumina (A1 2 O 3 ) 0.16

Insoluble residue 0.68

99.62%

Gypsum occurs mainly associated with stratified rocks, and
is regarded as a chemical deposit resulting from the evapora-
tion of waters of inland seas and lakes; it may also originate
through the decomposition of sulphides and the action of the

Geology of Ohio, 1888, Vol. VI, p. 700.

110 AQUEOUS ROCKS

resultant sulphuric acid upon limestone; through the mutual
decomposition of the carbonate of lime (limestone) and the sul-
phates of iron, copper, and other metals; through the hydration
of anhydrite; and through the action of sulphurous vapors and
solutions from volcanoes upon the rocks with which they come
in contact.

The gypsum deposits of northern Ohio form apparently con-
tinuous beds over thousands of square miles, and are regarded
by Professors Newberry and Orton as deposited by the evapo-
ration of landlocked seas at the same time as was the rock-salt
which overlies them.

Geological Age and Mode of Occurrence. As may be readily
inferred from what has gone before, beds of gypsum have
formed at many periods of the earth's history, and are still
forming wherever proper conditions exist.

In New York there are extensive deposits belonging to the
Salina period of the Upper Silurian. In Ohio, gypsum asso-
ciated with limestones and shales of Lower Helderberg age occur
over areas comprising thousands of square miles. The follow-
ing section of beds in Ottawa County, this state, will serve to
show the conditions under which the rock may occur :

Drift clays 12 to 14 feet

Gray rock carrying impure gypsum 5 to 14 feet

Blue shale 4 to 14 feet

Boulder bed carrying gypsum embedded in shaly limestone . 5 to 14 feet

Blue limestone 1 to 14 feet

Main gypsum bed 7 to 14 feet

Gray limestone 1 to 14 feet

Gypsum 3 to 5 feet

Anhydrite is an anhydrous variety of calcium sulphate some-
what less common than gypsum. Barite, or heavy spar, the
sulphate of barium, also occurs in nature, but less abundantly
than the calcium sulphates. It is found commonly in con-
nection with metallic ores (silver, lead, and zinc), or as a
secondary mineral associated with limestone, sometimes in
distinct veins, or, as in southwest Virginia, filling irregular
fractures in certain beds of the Cambrian limestones, or in
part replacing the limestone itself. It is easily distinguished
from coarsely crystalline calcite, for which it might possibly
be mistaken, by its weight, the specific gravity being about
4.5 as against 2.7 for the latter.

PHOSPHATES 111

(5) PHOSPHATES

The mineral apatite, a phosphate of lime, as already noted, is
a common accessory, in the form of small crystals, in crystal-
line rocks of all ages, both metamorphic and eruptive. In
rare instances, as among certain Laurentian rocks of Canada, it
occurs in coarsely granular aggregates of a green or pinkish
color and of such dimensions as* to constitute true rock masses.
Here we have to do, however, more with the amorphous, fibrous,
or concretionary forms to which the name phosphorite is com-
monly applied. These occur nearly if not quite altogether as
secondary products, due to the leaching out of phosphatic mate-
rial from older rocks, and its redeposition in clefts and cavities
at lower levels. It is thus that the phosphorites of Estre-
madura, Spain, are accounted for. From these very pure,
semi-crystalline masses, to the amorphous nodular and earthy
forms, such as are found in the eastern Carolinas, Florida and in
Tennessee, there are no well-defined lines of demarcation. All
have resulted apparently either from the leaching out of the phos-
phate as above, or from the dissolving and carrying away of the
lime carbonate in a phosphatic limestone, leaving the phosphatic
material to accumulate as a residual product. Some of the latter
products, like the phosphatic sandstones of the Carolinas, might
with equal propriety be classed with the fragmental rocks, as
are the residual clays. (See p. 112.)

(6) CHLOKIDES

Sodium chloride, or common salt, is one of the most wide-
spread constituents of the earth's crust, and from the standpoint
of human comfort a most important constituent as well. The
theoretically pure mineral consists of 66.6 parts of sodium and
39.4 parts of chlorine, though in nature it is almost univer-
sally contaminated with chlorines, sulphates, and carbonates
of potassium, calcium, and magnesium, together with oxides of
iron and aluminum. A large number of analyses of rock-salts
from world-wide sources show them to range from 94 to 99%
sodium chloride. The pure mineral is white in color, but
shows often yellow, red, or purplish hues due to iron oxides or
organic matter. When crystallizing freely from solution, it
ordinarily assumes the form of a cube, the faces being frequently
cavernous or hopper-shaped; rarely it occurs in octahedrons,
and occasionally in fibrous forms. Sodium chloride in solution

112 AQUEOUS ROCKS

is an almost universal constituent of terrestrial waters, though
often in but the merest traces. Its prevailing solid form is that
of coarsely granular aggregates constituting the so-called rock-
salt, the beds of which are often of such thickness and extent
as to constitute true rock masses and entitle them to considera-
tion here. These rock masses are invariably products of depo-
sition from solution, a deposition brought about through the
evaporation of saline waters in enclosed lakes or seas. They
are not limited to any particular geological period, but are to be
found wherever suitable conditions have existed for their for-
mation and preservation. Some of the more important beds
now known belong to either the Upper Silurian, Carboniferous,
Triassic, or Tertiary periods, and vary in thickness from a mere
film to upwards of 1200 feet. In the United States, beds of
rock-salt are known to occur in the states of New York, Penn-
sylvania, Ohio, Virginia, West Virginia, Michigan, Kansas,
Kentucky, Texas, Wyoming, California, and Nevada. Canada,
England, the Carpathian Mountains, the Austrian and Bavarian
Alps, West Germany, the Vosges, the Jura, Spain, the Pyrenees
and Celtiberian mountains, all contain important beds. With
the rock-salt are not infrequently associated other salts, as above
noted. In the celebrated Stassfurth deposits, sixteen different
compounds in the shape of chlorides and sulphates of sodium,
potassium, magnesium, calcium, and iron have been determined,
many of them in sufficient quantity to be of commercial value.

2. BOCKS FORMED AS SEDIMENTARY DEPOSITS AND FRAG-
MENT AL IN STRUCTURE: CLASTIC

The rocks of this group differ from those just described in
that they are composed mainly of fragmental materials derived
from the breaking down of older rocks, or are but the more or
less consolidated accumulations of organic and inorganic debris
from plant and animal life. The group shows transitional
forms into the last, as will be illustrated by certain of the lime-
stones and the quartzites. They are water deposits, and, as a
rule, are eminently stratified or bedded, although this structure
is not always apparent in the hand specimen.

As will be readily comprehended when one considers from
what a multitude of materials the fragmental rocks have been
derived, the amount of assorting, admixture with other sub-
stances, solution, and transportation by streams these materials

PLATE 10

FIGS. 1 and 2. Shell limestones. FIG. 3. Crinoidal limestone.

AEENACEOUS EOCKS: PSAMMITES 113

have undergone, they cannot be classified by any hard and fast
lines, but one variety may grade into another, both in texture
and structure as well as in chemical composition, almost indefi-
nitely. Indeed, many of them can scarcely be considered as
more than indurated muds, and only very general names can
be given them.

Accordingly as these rocks consist of mechanically formed
inorganic particles of varying composition and texture, or of
the more or less fragmental debris from plant and animal life,
they are here divided into two main groups, each of which is
subdivided as below:

I. Rocks formed by mechanical agencies, and mainly of in-
organic materials.

(1) The Arenaceous group Psammites: Sand, gravel, sand-
stone, conglomerate, and breccia.

(2) The Argillaceous group Pelites: Kaolin, clay, wacke,
shale, clayey marl, argillite.

(3) The Calcareous group Arenaceous and brecciated lime-
stones. The rocks of this group are often in part organic, and in
part chemical deposits. Only those are considered here in which
the fragmental nature is the most pronounced characteristic.

(4) The Volcanic group Fragmental rocks composed mainly
of ejected volcanic material: Tuffs, lapilli, sand and ashes,
pumice-dust, trass, peperino, pozzuolano, etc.

II. Rocks formed largely or only in part by mechanical agen-
cies and composed mainly of the debris from plant and animal
life.

(1) The Siliceous group Diatomaceous earth.

(2) The Calcareous group Fossiliferous and oolitic lime-
stone, marl, shell-sand, shell-rock.

(3) The Carbonaceous group Peat, lignite, and the coals.

(4) The Phosphatic group Phosphatic sandstone, guano,
coproiite nodules.

(1) EOCKS COMPOSED MAINLY OF INOEGANIC MATEEIAL.

(1) The Arenaceous Group: Psammites. Arenaceous, from
the Latin arenaceous, sandy or sand-like; psammite from the
Greek ^a/^urn??, sandy.

These rocks are composed mainly of the siliceous materials
derived from the disintegration of older crystalline rocks which
9

114

AQUEOUS ROCKS

have been rearranged in beds of varying thickness through the
mechanical agency of water. They are, in short, more or less
consolidated beds of sand and gravel. In composition and tex-
ture, they vary almost indefinitely. Many of them in which the
particles have suffered little during the process of disintegration

and transportation, are
composed of essentially
the same materials as the
rocks from which they
were derived. Others
have had the softer and
more soluble minerals re-
moved, leaving the sand
composed mainly of the
hard, almost indestruc-
tible mineral quartz.

In structure, the sand-
stones also vary greatly,
in some the grains being
rounded, while in others
they are sharply angular.
Figure 9 shows the mi-

FIG. 9. Microstructure of sandstone,
Portland, Connecticut.

croscopic structure of a brown Triassic sandstone from Portland,
Connecticut.

The material by which the individual grains of a sandstone
are bound together is as a rule of a calcareous, ferruginous, or
siliceous nature; sometimes argillaceous. The substance has
been deposited between the granules by percolating water or
during the process of sedimentation, and forms a natural
cement. It sometimes happens that the siliceous cement is
deposited about the rounded grains of quartz in the form of a
new crystalline growth, converting the stone into quartzite;
such are -in this work classed with the crystalline rocks.

Upon the character of this cementing material and the close-
ness with which the grains are bound together, is very largely
dependent the power of the stone to resist disintegration under
the trying action of percolating carbonated waters and the
mechanical action of heat and frost. The calcareous, and to a
less extent the ferruginous cements are liable to removal in
solution, allowing the rock to fall away to sand, or at least

ARENACEOUS ROCKS: PSAMMITES

115

allowing it to absorb water, which, on freezing, brings about
the disintegration. The argillaceous cementing material, while
in itself inert, also permits a high degree of absorption, with
like results. Those sandstones cemented by silica, which there-
fore partake of the nature of quartzite (see p. 137), are by far
the more refractory.

The following analyses will serve to indicate the consid-
erable range in composition of rocks of this class:

CHEMICAL COMPOSITION OF SANDSTONES

CONSTITUENTS

III

Silica (Si0 2 )

69.94%

84.40%

95.24%

90 86 %

Alumina (AlgOs)

13.15

7 49

4 76

Iron oxides (Fe 2 O 3 ) and (FeO) . .
Manganese (MnO)

2.48
0.70

3.87

1.28

1.58

Lime (CaO)

3.09

0.74

1.40

Magnesia (MgO)

Trace

2.11

1.23

Potash (K 2 0)
Soda (Na 2 O)

3.30
5 43

0.24
56

1.06
45

Loss ...

1 01

Totals .

99. 10 %

99 41 L

100 27 %

99 45 /

I. Brown Triassic sandstone: Portland, Connecticut. II. Gray sub-Carbo-
niferous sandstone: Berea, Ohio. III. Red Carboniferous sandstone: Anan,
Scotland. IV. Cambrian sandstone: Siskowit Bay, Wisconsin.

The table given on p. 145 will serve to show the close chem-
ical relationship existing between many rocks of this group,
and their metamorphic equivalents.

The colors of sandstone are dependent upon a variety of
circumstances. The red, brown, and yellowish colors are due
to iron oxides in the cementing constituent. Some of the dark
colors are due to carbonaceous matter.

Many varieties of sandstone are popularly recognized. Cal-
careous, ferruginous, siliceous, or argillaceous sandstones are
those in which the cementing materials are of a calcareous, fer-
ruginous, siliceous, or argillaceous nature. The name arkose is
given to a coarse feldspathic sandstone derived from granitic
rocks, with a minimum amount of loss of original material. Con-
glomerate or puddingstone is merely a coarse sandstone ; it differs
from ordinary sandstone only as gravel differs from sand. Brec-

116 AQUEOUS BOOKS

cia is a fragmental rock differing from conglomerate in that the
individual particles are sharply angular instead of rounded.
The term is made to include also certain volcanic rocks with a
brecciated structure. (See PL 4.)

Greywacke or Grauwacke is an old German name for brecci-
ated fragmental rocks made up of argillaceous particles. The
name is now little used. Other names, as flagstone, freestone, and
brownstone, are applied to such as are used for flagging or other
structural purposes. Itacolumite is a feldspathic sandstone, or
perhaps more properly quartzite, in which the feldspathic mate-
rial plays the role of a binding constituent to the quartz gran-
ules. The so-called flexible sandstone is an itacolumite from
which the feldspathic portions have been removed by decompo-
sition leaving the interlocking quartz grains with a small amount
of play between them. The rock is in no sense elastic, but
merely loose jointed.

The name greensand, greensand marl, and glauconitic sand are
given to a prevailing dull green, loosely coherent, clayey or
arenaceous deposit which owes its peculiarities to the presence
of the hydrous silicate of iron and potassium glauconite, but
which is variously contaminated with minute particles of quartz
and siliceous minerals, the iron oxides, clay, rock fragments,
and particles of shells.

The analyses on page 117 are from the Report of the Geo-
logical Survey of New Jersey, for 1892. 1

The most extensive and best known deposits in the United
States are included in what are known as the Upper, Middle,
and Lower marl beds of the Cretaceous formations in south-
eastern New Jersey.

Rocks belonging to the arenaceous group are world wide
in their distribution. Being themselves the products of disinte-
gration and decomposition of pre-existing rocks, and having
become consolidated under conditions not greatly different from
those now existing at or near the surface of the earth, the rocks
of this group are as a whole in a state of comparatively stable
chemical equilibrium. Unless including calcareous matter or
readily oxidizing ferruginous compounds, such are subject to

lr The reader is referred to Professor W. B. Clarke's paper on " The
Cretaceous and Tertiary Formations of New Jersey/' in the Ann. Rep.
State Geologist of New Jersey for 1892.

ARGILLACEOUS EOCKS: PELITES

117

disintegration more through physical than chemical agencies, as
will be noted later.

CHEMICAL COMPOSITION OF GLAUCONITIC MARLS

CONSTITUENTS

III

VII

Phosphoric acid. . . .

-lie

A CO

%
1 ^1

ft IQ

O en

fi ft7

q 7q

Sulphuric acid.

1 9ft

9 4O

ft 41

n .4.

q 1 o

9 4.4.

Silica, and sand ....

04 K(\

4K rn

CK fiQ

C1 1 C

47 ^O

44 fift

4Q fift

Potash

1 ^4

q 70

c 97

"7 Oft

F; OQ

q 07

4 QR

Lime

9 *9

1 ^1

fi^

n 4Q

^fi

4 Q7

4 14

Magnesia

21 c

o on

ft 7Q

9 09

9 7ft

9 Q7

A 47

Alumina

fi no

c on

fi fii

ft 9i

R fiO

fi 04

Oxide of iron

O1 Cf\

94. f^rt

91 fi^

oq -i q

9O ^9

1ft Q7

9ft 71

Water

1ft ftft

1 ^ 4ft

C Q

fi fi7

to C7

ft fi .

C K4

99.43

99.18

102.40

99.37

99.58

99.32

99.69

I. Clay marl, from near ^.lattawan. II. Clay marl, from Matchaponix
Creek, three miles south of Spottswood. III. Lower marl, from Navesink
Highlands. IV. Middle marl, from near Eatontown. V. Middle marl, from
southeast of Freehold. VI. Upper marl, from Poplar. . VII. Upper marl,
from Shark Eiver.

(2) The Argillaceous Group: Pelites. The rocks of this
group are composed of more or less hydrated aluminous sili-
cates admixed in almost indefinite proportions with siliceous
sand, various silicate minerals in a more or less fragmental and
decomposed condition, and calcareous and carbonaceous matter.
In their least consolidated form they are best represented by
the common plastic clays used for brick and pottery manufac-
ture. Such, although alike in their general physical or even
ultimate chemical nature, have widely diverse origins. In fact,
the term day, like silt, indicates physical condition rather than
chemical or mineralogical composition, and it may perhaps be
defined as an indefinite admixture consisting largely of more or
less 'hydrated aluminous silicates and free silica, with lesser
amounts of iron oxides, carbonates of lime, and various silicate
minerals which in a more or less decomposed and fragmental
condition have survived the destructive agencies to which they
have been subjected. About the only feature characteristic of
all clays, is that of plasticity, when wet, and this is dependent,

118 AQUEOUS EOCKS

apparently, upon texture and structure, i. e., upon the size and
shape of the individual particles, and in some cases at least
the presence of colloidal matter. Pure quartz, chalcedony, flint,
feldspar, or other silicates, will, when reduced to an impalpable
powder, possess the qualities usually ascribed to clay, and in the
pages following, the term is used only with reference to degree
of comminution and plasticity, regardless of mineral nature or
chemical composition. It includes residual products of any or
all forms of rock degeneration, and which may or may not have
been reasserted through the agency of water. ( See further under
The Eegolith, Part V.) The oft-repeated statement that kaolin
forms the basis of clays, or that clay is impure kaolin, is an
unfounded assumption, and if accepted at all it must be with the
reservations made by Johnson and Blake, 1 who limit the term
kaolin to the impure material, quite distinct from true kaolinite,
which is a definite chemical compound corresponding to the
formula H 4 Al 2 Si 2 9 .

Throughout the glaciated region of the northeastern United
States the clays are largely glacial silts or water deposits from
the floods of the Champlain epoch. The latter are often beauti-
fully and evenly stratified, as shown in the illustration on PL 28.
The plastic clays and siliceous sands about Woodbridge, New
Jersey, are regarded as derived from the Azoic rocks and de-
posited by sea-water in enclosed basins. The exact source of
the material is not always apparent; the porcelain clays of Law-
rence County, Indiana, on the other hand, are, according to
State Geologist Cox, residual deposits resulting from the de-
composition of impure Carboniferous (Archimedes) limestones,
the lime carbonate being removed in solution, while the less
soluble clay remains. Kaolin, as already noted, is a residual
deposit from the decay of feldspathic and other aluminous rocks,
while the ordinary brick and tile clays of the Southern states, as
well as the clayey soils, are residual aluminous deposits resulting
from the decay and leaching out of soluble constituents from a
variety of rocks, both sedimentary and eruptive. (See chapter
on rock weathering.)

As showing the comparative compositions of kaolins and other
clays, the following table is given:

1 Am. Jour, of Science, 1867, p. 351.

ARGILLITES AND SHALES 119

CHEMICAL COMPOSITION OF KAOLINS AND OTHER CLAYS

CONSTITUENTS

III

Si0 2 (combined) . .
Si0 2 (free) . .

46.4 %

39.00%

34.70o/
12.20

28.30 %
27.80

42.71 %
0.70

J60.97 %

A1 2 3

39.7

36.00

31.34

27.42

39.24

26,38

H 2 O (combined) . .
H 2 0at212

13.9

14.00
9.50

12.00
8.00

6.60
2.90

13.32
1.58

} 8.93

CaO and MgO . . .

0.63

0.10

0.18

0.20

Alkalies

054

2 71

1.90

FeoOs

2 68

146

99.00 %

99.67o/

9945o/

98.59o/

99.10%

99.64%

I. Kaolin. IT. Indianite, a white clay residual from St. Lawrence County,
Indiana III Potter's clay, from Pope County, Illinois. IV. Brick clay
from New Jersey. V Fire clay from New Jersey. VI Fire clay from
Illinois.

Amongst the older formations the clays have largely under-
gone induration, giving rise to what are known as argillites, or
if fissile, slates or clay slates, such as are used for roofing and
similar purposes, the fissile property having been imparted by
pressure or shearing. Such forms pass by imperceptible grada-
tions into argillaceous schists which are classed with the meta-
morphic rocks. (See p. 135.) The argillites are, as a rule,
among the most indestructible of rocks, since they are them-
selves composed of the least destructible debris of pre-existing

CHEMICAL COMPOSITION OF ARGILLITES.

CONSTITUENTS

Silica (Si0 2 )

58 37 %

60.32%

60.150%

Sulphuric acid (H 2 S0 4 )
Alumina (Al 2 0s)

0.22
21.985

23.10

24.20

Iron oxides (FeO) and (Fe 2 3 ) . .
Lime (CaO)
Magnesia (MgO)
Soda (Na 2 O) .
Potash (K 2 O)
Water (H 2 0)

10.661
030
1.203
1.933

4.03

7.05

0.87
049
3.83
4.08

7.65

1 4.278
3.72

98.702%

99.74%

99.998%

120

AQUEOUS ROCKS

rocks. Their ultimate chemical composition is much like that
of the clays, and scarce any two samples will show similar results
when submitted to analysis. The table given on page 119 shows
the composition of some schistose argillites used for roofing pur-
poses from (I) Harford County, Maryland, (II) Lancaster
County, Pennsylvania, and (III) Llangynog, North Wales.

Shale is a somewhat loosely defined term, indicating struc-
tural rather than chemical or mineralogical composition. The
word is perhaps best used in its adjective sense, as a shaly
sandstone, or shaly limestone. By many authors it is used
with reference more particularly to thinly stratified or lami-
nated, clayey rocks. Many shales are but the finer, more fissile
portions of sandstone beds; such may represent the off-shore
quiet or deep-water portions of arenaceous sediments, which, be-
ginning with gravels near the shore-line, become gradually finer
as the distance from the shore increases, passing through coarse
to finer sands and finally to sandy clays and silts as the water,
through the lessening of its carrying power, lays down its load.
Or they may represent later stages in the cycle of sedimenta-
tion; the finer silts brought down after erosion have so far

CHEMICAL COMPOSITION OF SHALES

CONSTITUENTS

III

Silica (SiO 2 )

50.13%

72.40%

66 96 %

Alumina (A1 2 3 )
Iron sesquioxide (Fe 2 3 )
Lime (CaO)

10.73

578
40

16.45
1.05
17

15.626

8.38
493

Magnesia (MgO) .

1.00

1 48

677

Potash (K 2 0)

5.08

3 295

Soda (Na 2 O)

0.53

0.628

Sulphur (S) .

4 02

1 21

Carbon (C)

22 83

Undet

3 787

Water (H 2 O) l

2.21

Undet

Phosphoric acid (P 2 5 )

154

97.10%

9837%

100.00 %

I. An alum shale from Garnsdorf, near Saalsfeld. II. An alum shale
from Barnholm. III. A " marly shale " from Breckenridge County, Ken-
tucky^

1 Ignition.

CALCAKEOUS FEAGMENTAL KOCKS 121

reduced the level of the land as to greatly diminish the currents
and consequent carrying power of the seaward-flowing streams.
Such beds, on consolidation, yield then what are commonly
known, in the order of their formation, as conglomerates, sand-
stones, shales and argillites, or clay slates, the shales occu-
pying, both in texture and composition, a position intermediate
between the argillites and sandstones.

The table on page 120 shows the varying character of the rocks
included under this name. Those given in columns I and II carry
sulphur in combination with iron, as iron pyrites (FeS 2 ). This,
on decomposing, through the action of meteoric waters, yields
iron sesquioxides and sulphuric acid, the latter combining with
a portion of the alumina in the rock to form sulphate of alumi-
num, or common alum. Hence they have been called alum shales.

Laterite is a red, ferruginous residual clay found in tropic
and semitropic regions. (See p. 298.) Catlinite, or Indian
pipe-stone, is an indurated clay rock formerly used by the Da-
kota Indians for pipe material. The name porcellanite has
been given to a compact porcelain-like rock consisting of clay
indurated by igneous agencies. The name wacke is sometimes
used to designate an earthy or compact, dark-colored clayey
material resulting from the decomposition in situ of basaltic
rocks. Adobe is the name given to a calcareous clay of a
general gray-brown or yellowish color, very fine grained and
porous, and which is widely distributed throughout the more
arid regions of the West. It is described in greater detail
under the head of Soils (p. 320). Loess is a somewhat similar
material forming the surface soil over wide areas in the Missis-
sippi valley, and at times sufficiently plastic for brick making.
(See also p. 315.)

(3) The Calcareous Group. Here are brought together a
small series of fragmental rocks composed mainly of calcareous
material, but of which the organic nature, if such it had, is not
apparent. These rocks form at times beautifully brecciated
marbles. Their structure may be best comprehended by remem-
bering that the original beds, whether crystalline or amorphous,
whether fossiliferous or originating as chemical precipitates,
have been crushed and shattered into fragments, and then, by
infiltration of lime and iron-bearing solutions, slowly cemented
once more into firm rock. The composition is essentially the same
as the ordinary sedimentary limestones and need not be further

122 AQUEOUS KOCKS

dwelt upon here It may be stated, however, that owing to the
softness and ready solubility of their materials limestones do not,
on breaking down, except under rare instances, give rise to ex-
tensive beds of arenaceous rocks, as do the siliceous varieties.
One of the best known rocks of this group is the breccia marble
near Point of Rocks in Maryland, which has been used for deco-
rative purposes in the Capitol building at Washington.

(4) The Volcanic Group: Tuffs Under this head are in-
cluded a great variety of fragmental rocks, composed of the
more or less finely comminuted materials ejected from vol-
canoes as ashes, dust, sand, and lapilli. These occur, in many
instances, interbedded with lava flows of the same lithological
nature, and are a product of the same periods of volcanic
activity, the eruption of molten lava being interrupted by
intervals of explosive action, during which only fragmental
material was ejected. To such materials the name pi/roclastic
(Greek TT^O?, fire) is appropriately given.

The lithological character of the materials varies greatly, and
only very general names are given them in the majority of cases.
The name tuff or tufa is given to the entire group formed as
above, and also by some authorities to fragmental rocks resulting
from the breaking down and reconsolidation of older volcanic
lavas. It would seem advisable to designate these last, as has F.
Lowinson-Lessing', 1 as pseudotuffs or tuffoids.

The names volcanic ashes, sand, and dust are applied to
the finer of these volcanic materials in an unconsolidated state
and lapilli or rapilli to the coarser fragments.

The dusts and sands are not infrequently composed of
minute shreds of volcanic glass, which were blown from the
volcanic vents and carried unknown distances, to be ultimately
deposited as stratified beds in comparatively shallow water.
Such are described more in detail under the head of JSoliaii
rocks (p. 133). The term trass is used to designate a compact
or earthy fragmentai rock composed of pumice dust, in which
are embedded fragments of trachytic and basaltic rocks, car-
bonized wood, etc., and which occupies some of the valleys of
the Eifel. Peperino is a tufaceous rock composed of fragments
of basalt, leucite lava, and limestone, with abundant crystals
of augite, mica, leucite, and magnetite. It occurs among the
Alban Hills, near Eome, Italy. Palagonite tuff is composed of

1 Tschermaks, Min. u. Petrog. Mittheilungen, Vol. IX, 1889, p. 530.

VOLCANIC TUFFS

123

dust and fragments of basaltic lava, with pieces of a pale yellow,
green, reddish, or brownish glass called palagonite. The general
name of volcanic mud is given to the finely comminuted volcanic
material which in a more or less pasty or liquid condition is
thrown from volcanic vents during the incipient stages of
eruption.

The tuffs are as a rule more or less distinctly stratified and of
very uneven texture. They are found associated with volcanic
rocks of all ages, and are at times so highly metamorphosed as to
render the original nature of some doubt. Certain English
authorities have contended that a part of the so-called argillites
and fire clays were of finely comminuted volcanic materials.

The composition of the tuffs naturally varies with that of the
character of the lava from which they were derived. Being often
porous and readily permeated by water or rootlets, they undergo
decomposition, forming soils the character of which is dependent
to some extent upon their lithological nature. The following
table shows the varying composition of rocks of this class :

CHEMICAL COMPOSITION OF VOLCANIC TUFFS

KINDS AND LOCALITIES

<u O
fl be

Eft|

a 2-

Pozzuolana, Naples,
Italy ....

59.144

21.28

4.76

1.90

*..

4.37

6.23

97.68

Tuff, Crater of Monte

Nuova, Italy . .

56.31

15.23

7.11

1.74

1.36

6.54

4.84

6.12

99.25

Trass, Andernach,

v v '

Prussia. . . .

54.00

16.50

6.10

4.00

0.70

10.00

7.00

98.20

Tuff, Lacher See,

Prussia ....

60.49

19.95

9.37

3.12

1.43

3.40

1.33

99.09

(2) ROCKS COMPOSED MAINLY OF DEBRIS FROM PLANT AND

ANIMAL LIFE

(1) The Siliceous Group: Diatomaceous Earth. This is a
fine white or gray pulverulent rock, composed mainly of the
minute shells, or tests, of diatoms, and often so soft and friable
as to crumble readily between the thumb and fingers. It occurs

AQUEOUS EOCKS

in beds which, when compared with other rocks of the earth's
crust, are of comparatively insignificant proportions, but which
are nevertheless of considerable geological importance. Though
deposits of this material are still forming, and have been formed
in times past at various periods of the earth's history, they
appear most abundantly associated with the Tertiary formations.
The beds are wide-spread, and some of them of economic
importance. A deposit in Biln, Bohemia, is some 14 feet in
thickness, and is estimated by Ehrenberg to contain 40,000,000
shells to every cubic inch. Beds occur in the United States at
South Beddington, Maine; Lake Umbagog, New Hampshire; in

FIG. 10. Section through lake basin showing the formation of diatomaceoua
earth, a, bed rock; fcfc, floating peat; cc, decayed peat; d, diatomaceous
earth.

Morris County, New Jersey; near Richmond, Virginia; in Cal-
vert and Charles counties, Maryland; in New Mexico; Graham
County, Arizona ; near Reno, Nevada, and in various parts of
California and Oregon.

Chemically the rock is impure opal, as shown by the following
analyses of samples from (I) Lake Umbagog, New Hampshire,
(II) Morris County, New Jersey, and (III) Pope's Creek, Mary-
land:

CHEMICAL COMPOSITION OF DIATOMACEOUS EARTH.

CONSTITUENTS

III

Silica (SiO 2 )

80.53%

80.60%

81.53%

Iron oxides (Fe 2 8 and FeO) . . .
Alumina (A^Og) .

1.03
5 89

3 84

3.33
3 43

Lime (CaO) ....

0.35

2 61

Water (H 2 0)

11.05

14.00

6 04

Organic matter

0.98

99.83 %

99.02%

96.94%

Number III showed also small amounts of potash and soda.

(2) The Calcareous Group. These rocks are made up of
the more or less fragmental remains of molluscs, corals, and

PLATE 11

,.* *~

*wv*,v*i

^;* l \..>^' *** M

^W^

*" ^'^^^Nfi*

FIG. 1. Pisolitic limestone.

FIG. 2. Oolitic limestone.

LIMESTONES 125

other marine and fresh-water animals. Many of them are but
consolidated beds of calcareous mud, full of more or less frag-
mentary shells or casts of shells, as shown in Fig. 1, PL 10. The
name coquina is given to such as that shown in Fig. 2, PL 10,
from St. Augustine, Florida. The rock is composed almost
wholly of very perfect shells of a bivalve mollusc, loosely ce-
mented by calcareous materials in a finely divided condition.
Special names are given these calcareous rocks, designating the
character of materials from which they are derived. Coral and
shell limestones, as the names denote, are composed mainly of the
debris from these organisms. In like manner such names as
crinoidal, fusulina, etc., are applied.

Nummulitic limestone contains fossil nummulites. Rocks of
this type were used in the construction of the pyramids of
Cheops. Chalk is a fine-grained, white, pulverulent rock, com-
posed of finely broken shells of marine molluscs, among which
minute foraminifera are abundant. Shell sand is a loose aggre-
gate of shell fragments, formed on sea-beaches by the action of
the winds and waves. On certain Hawaiian beaches, such sands
give out a distinct note, or peculiar crunching sound when
walked over, or even when shaken in a closed vessel, and are
popularly known as sounding, or singing, sands. The property
is manifested only when the sand is dry and is assumed to be
due to the minute air cavities enclosed by the shells. Oolitic
and pisolitic limestones, as previously noted, are made up of
rounded concretionary masses of calcium carbonate, and are
in part of mechanical origin, and in part chemical deposits
(PI. 11).

The microscopic structure of an oolitic limestone from Prince-
ton, in Caldwell County, Kentucky, is shown in the accompany-
ing figure (p. 126). It will be noticed that the first step in the
formation of this stone was the deposition of concentric coat-
ings of lime about a nucleus which is sometimes nearly round,
but more frequently quite angular and irregular. After the
concretions were completed there were formed about each one,
narrow zones of minute radiating crystals of clear, colorless
calcite; then the larger crystals formed in the interstices. The
nuclei are composed in some cases of single fragments or, again,
of a group of fragments. Recent microscopic studies have tended

126

AQUEOUS EOCKS

RG. ll.-Microstructure of oolitic limestone.

to show that many of the oolitic limestones owe their structure
to the lime-secreting power of microscopic algae. 1

Limestones vary almost indefinitely in structure and color.

From the soft tufaceous
or highly fossiliferous
varieties there is a con-
stant gradation to dense
compact rocks breaking
with a conchoidal or
splintery fracture the
true nature of which is
sometimes to be ascer-
tained only by chemical
tests. There is a like
variation in color. White
through all shades of
gray to black are common,
and more rarely occur

y e " OW > brOWD > P ink ', r
red varieties, the colors

depending on organic matter and ferruginous oxides.

Owing to the readiness with which calcium carbonate under-
goes crystallization, even at ordinary temperatures, few lime-
stones are wholly amorphous, but grade insensibly into holo-
crystalline varieties such as are classed with the metamorphic
rocks. The name marble is given to such limestones as are of
sufficiently close texture to take a polish and of such colors as
to make them desirable for ornamental work. A large proportion
of the marbles belong, however, to the metamorphic group.
(See p. 141.) Figure 12 shows the microscopic structure of a
dark gray, variegated, highly fossiliferous limestone belonging to
the Cincinnati group, near Hamilton, Ohio. It is a natural result
of their method of formation that few limestones are of pure cal-
cium carbonate. A portion of the calcium is often replaced by
magnesium, giving rise to magnesian limestone, or to dolomite.
This last can as a rule be distinguished from limestone only by its
increased hardness (3.5-4.5) and specific gravity (2.8-2.95).
Frequently chemical tests are necessary, limestone effervescing
readily when treated with dilute hydrochloric acid, while dolo-
mite is unacted upon.

1 American Geologist, Vol. X, No. 5, 1892.

LIMESTONES

127

Mechanically included materials, as sand and clay, are com-
mon, giving rise to siliceous and argillaceous varieties. The so-
called hydraulic limestone
is one containing 10%
and upwards of these
impurities, and which
when burnt and ground,
forms a cement character-
ized by its property
of setting under water.
Many limestones, like the
dolomitic varieties in Cook
County, Illinois, contain
so large a proportion of
bituminous matter as to
give off a distinct odor of
petroleum when struck
with a hammer, or even to
become blackened on the
surface by its exudation
when exposed to the weather. Others contain phosphatic matter,
and pass by insensible gradations through what are known as
phosphatic limestones to true phosphates (phosphorites, etc.).

CHEMICAL COMPOSITION OF LIMESTONES AND DOLOMITES

FIG. 12. Microstructure of fossiliferous
limestone.

III

1 A

*il

o a.2

Iff

CONSTITUENTS

Bjjrf

^*-2f 8

SS*

gj-

|f||

d ~w fc

III

|||

^aa

gf$S

alii

fill

OPQ-S

Carbonate of lime (CaCO 3 ) . . .

98.00 %

54.62 %

41.88 %

72.95^

96.60 %

Carbonate of magnesia (MgC0 3 ) .

45.04

24.55

3.84

0.13

Oxides of iron (FeO and Fe 2 O 3 ) .
Oxide of aluminum (A1 2 O 3 ) . .

....

J0.23

| 4.03

1.34
4.50

0.98

Silica (SiO 9 ) 2 and insol. silicates .

0.57

....

29.93

14.79

0.50

Potash (K 2 O)

0.22

0.31

Soda (Na 2 O)

1.12

0.40

Water (H 2 O) . .

0.96

Sulphate of lime (CaSO 3 ) . . .

....

1.75

Organic matter .

1.46

Totals .... ....

98.57 %

99.89 %

101.73^

100.63^

99.88 %

128

AQUEOUS BOOKS

In chemical composition the limestones, like other sedimentary
rocks, vary greatly. As a general rule, those varieties, which
have been formed in deep waters and at a distance from the
shores, will be of greatest purity, since less likely to have be-
come contaminated through detrital materials washed in from
the land. Even these may, however, be intermingled to a very
considerable extent with the fine siliceous and ferruginous mat-
ters such as deep-sea dredgings have shown to be common to
modern sea-bottoms. The table on page 127 will give some idea
of the wide range in chemical composition found in rocks of
this class.

The name shell marl, or merely marl, is given to an illy defined,
often arenaceous, soft and earthy rock consisting essentially of
shell material in a more or less fragmental condition, and usu-
ally intermixed with more or less clayey matter or siliceous
sand and silt. Geikie 1 would limit the term to fresh-water
accumulations of remains of mollusca, entomostraca, and fresh-
water algas, but unfortunately the word has not been so used
in much of the literature extant. These marls, being easily
decomposed, and on account of their occasional richness in
phosphoric acid, or, perhaps, merely on account of the lime
they contain, are of value as fertilizers. The analyses below

CHEMICAL COMPOSITION OF MARLS

CONSTITUENTS

III

VII

Silica (SiO 2 )

6 97

61 61

18 84

58 25

25 28

39 36

5 65

Oxide of iron and alumina

(A1 2 3 - Fe 2 3 ) . . . .

0.86

2.80

2.72

11.28

3.02

3.47

3.30

Lime (CaO) ....

47 f?o

1Q fiO

41 4S

10 4Q

07 co

00 Qfi

AQ C1

Magnesia (MgO) ....

1.03

0.12

1 96

Potash (K,O)

2*3

Soda (Na.,O)

017

Phosphoric acid (P 2 O 5 )

0.19

0.18

0.40

0.11

trace

Sulphuric acid (SO 2 ) . .

0.41

0.06

0.64

040

0.18

0.31

Carbonic acid (CO 2 ) . .

38.15

15.37

32.07

10.59

29.02

22.73

39.80

Organic matter and water

(C and H 2 O) ....

4.25

3.42

....

2.98

4.11

0.60

Totals

99.00

99 44

100 00

93.61

99 21

100 00

100 66

1 Text-book of Geology, 3d ed.

PEAT AND LIGNITE

129

of North Carolina marls, consisting largely of comminuted
shells and sometimes coprolite nodules, will serve to show the
widely varying character of the materials grouped under this
name. 1

(3) The Carbonaceous Group: Peat, Lignite, and Coal.
Here are included a variety of more or less oxygenated hydro-
carbons varying widely in physical and chemical properties, but
alike in originating from decomposing plant growth protected
from the oxidizing influences of the air. Plants, when decom-
posing upon the surface of the ground, give off their carbon to
the atmosphere in the shape of carbonic acid gas (C0 2 ), leaving
only the strictly inorganic or mineral matter behind. When,
however, protected from this oxidizing influence by water, or
other plant growth, decomposition is greatly retarded, varying
portions of the carbonaceous and volatile matters are retained,
and the material becomes slowly converted into coal. Accord-
ing to the amount of change that has taken place in the original
plant material, the amount of volatile matter still retained by it,
its hardness and burning qualities, several varieties are recog-
nized, which, however, pass into each other by insensible grada-
tions.

Peat results from the gradual accumulation in bogs and
marshes of growths consisting mainly of sphagnous mosses, a low
order of plants having the faculty of continuing in growth
upwards as they die off below. In this way deposits often
assume a very considerable thickness. Where sufficiently thick,
the lower portions have sometimes been converted into a dense
brownish black mass somewhat resembling true coal. The
deposits of peat are all comparatively recent and occur only
in humid climates. They are developed to an enormous
extent in Ireland, and are also abundant in Europe and various

CONSTITUENTS

61.04 %

23.86 %

21.00 %

Volatile matter

37.53

56.13

72.00

Ash

1.83

19.77

7.00

Totals

100.40 %

99.76 %

100.00 %

1 Geology of North Carolina, Vol. I, 1875, p. 195.
10

130

AQUEOUS EOCKS

parts of North America. An impure variety containing a con-
siderable quantity of siliceous sand, and locally known as
"muck, " is used as a fertilizer and for mulching throughout
New England. On page 129 are given the results of analyses of
(I) peat from the bog of Allan, Ireland, (II) Commander
Islands in Bering Sea, and (III) Maine (United States).

Lignite, or brown coal, is the name given to a brownish black
material characterized by a brilliant lustre, conchoidal fracture,
and brown streak. Such contain from 55% to 65% of carbon,
and burn easily, with a smoky flame, but are inferior to the true
coals for heating purposes.

Bituminous Coal. Under this name are included a series of
compact and brittle products in which no traces of organic
remains are to be seen on casual inspection, but which, under
the microscope, often show traces of woody fibre, spores of
lycopods, etc. These coals are of a brown to black color, with
a brown or gray brown streak, break with a cubical or conchoidal
fracture, and burn readily with a yellow, smoky flame. They
contain from 35% to 70% of fixed carbon, 18% to 60% of vola-
tile matter, and from 2% to 20% of water, and only too fre-
quently show traces of sulphur due to included iron pyrites.
Several varieties of bituminous coals are recognized, the distinc-
tions being based upon their manner of burning. Coking coals
are so called from the facility with which they may be made
to yield coke. Other varieties of apparently the same com-
position and general physical properties, cannot, for some
unexplained reason, be made to yield coke, and are known
as non-coking coals. Cannel coal has a very compact struc-
ture, breaks with a conchoidal fracture, has a dull lustre,

CHEMICAL COMPOSITION OF COALS

CONSTITUENTS

Water

1.105%

Volatile matter

29.885

58.00%

Fixed carbon

57.754

23.50

Ash

9.895

18.50

Sulphur

1.339

99.978#

100.00%

THE PHOSPHATES 131

ignites easily, and burns with a yellow flame. On the opposite
page is given the composition of (I) a coking coal from the Con-
nelsville Basin of Pennsylvania, and (II) a cannel coal from
Kanawha County, West Virginia. 1

Anthracite Coal. This is a deep black, lustrous, hard and
brittle variety, and represents the most highly metamorphosed
variety of the coal series. Such have been generally regarded
as bituminous coals from which a very large proportion of the
volatile constituents have been driven off by the agencies in-
volved in the production of mountain systems by the heat inci-
dent to the injection of igneous rocks, or through the oxidizing
influence of percolating water. Below is given the average
composition of anthracite from the Kohinoor Colliery, Shenan-
doah, Pennsylvania.

Water 3.163%

Volatile matter 3.717

Fixed carbon 81.143

Sulphur 0.899

Ash 11.078

100.000%

The principal anthracite coal regions of the United States are
in eastern Pennsylvania. From here westward throughout the
interior states to the front range of the Kocky Mountains the
coals are all soft, or bituminous coals. Those of the Rocky
Mountain regions proper are largely lignitic, passing into the
bituminous varieties.

(4) Phosphatic Group: Phosphatic Sandstone; Bone Breccia;
Guano ; Coprolite Nodules. This is a group of rocks limited in
extent, but nevertheless of considerable economic importance,
owing to the high values of certain varieties for fertilizing pur-
poses. Guano consists mainly of the excrement of sea fowls, and
is to be found in beds of any importance only in rainless regions
like those of the western coast of South America and southern
Africa. The most noted deposits are on small islands off the
coast of Peru. Immense flocks of sea fowls have, in the course
of centuries, covered the ground with an accumulation of their
droppings to a depth of sometimes 30 to 80 feet, or even more.

An analysis of American guano gave: Combustible organic

1 F. P. Dewey, Bull. 42, U. S. National Museum, 1891.

132 AQUEOUS EOCKS

matter and acids, 11.3%; ammonia (carbonate, etc.), 31.7%;
fixed alkaline salts, sulphates, phosphates, chlorides, etc., 8.1% ;
phosphates of lime and magnesia, 22.5% ; oxalate of lime, 2.6% ;
sand and earthy matter, 1.6%; water, 22.2% (Geikie). Copro-
lite nodules are likewise the excrements of vertebrate animals;
those among the Carboniferous shales of the basin of the Firth
of Forth are regarded as accumulated excretions of ganoid fishes.
Phosphatic sandstones, as the name denotes, are arenaceous
rocks containing more or less phosphatic matter. Inasmuch as
the phosphatic material is derived largely by leaching and
segregation, these rocks have been already described under the
head of chemical deposits (p. 111). In the river beds of the
Eastern Carolinas are found rounded and nodular masses of this
nature, consisting of siliceous and calcareous sand, with em-
bedded bones, teeth of sharks, and other animal remains. Bone
breccia consists of fragmental bones of mammals cemented by
argillaceous, earthy, or calcareous matter.

III. ^OLIAN ROCKS

This group comprises a small and comparatively insignificant
class of rocks formed from materials drifted by the winds, and
more or less compacted into rock masses. They are, as a rule,
of a loose and friable texture and of a fragmental nature.
Many of the volcanic fragmental rocks (tuffs) are grouped here.

One of the most common results of wind action on the land
is the production of sand-dunes billowy masses of loose sand
which, like drifts of snow, gradually change their outlines and
creep onward under the restless goading of the wind.

Such, owing to their superficial nature, recent origin, and
loose state of consolidation, are considered more in detail in
the chapter on The Eegolith, p. 287. On undergoing consoli-
dation, these dune sands may give rise to sandstones in many
instances indistinguishable from those of aqueous origin, though
less regularly bedded. The finely disintegrated shell and coral
sand thrown up by the waves on the beaches of Bermuda is
caught up by the winds and drifted inland, forming hills which,
in some instances, are 250 feet in height. Through the deposi-
tion of lime carbonate in the interstices of the fragments, these
become reconsolidated and form thus the drift rock which com-
prises a large portion of the mass of the islands above tide level.

The finely comminuted materials ejected from volcanic vents
may be likewise transported by atmospheric currents and, far
from their source, again deposited in beds of no insignificant
proportions. These, on induration, give rise to fine-grained tuffs,
and, where the final deposition has taken place in water, to
distinctly laminated, fine white rocks the lithological nature
of which can be made out only by means of the microscope.
Such are many of the Pliocene sandstones of Idaho and Mon-
tana. 1 ( See Fig. 2, PL 28. ) The following analyses of samples of
pumiceous tuffs from (I) Marsh Creek Valley, Idaho; and (II)
Little Sage Creek, Montana, will serve to show their composition.

1 On the Composition of Certain Pliocene Sandstones from Montana and
Idaho, Am. Jour, of Science, Vol. XXVII, 1886, p. 199.

133

134

AEOLIAN BOCKS
CHEMICAL COMPOSITION OF VOLCANIC DUST

CONSTITUENTS

Ignition (H20) .

7.60 %

7 62 %

Oxide of iron and aluminium (Fe 2 Os and A1 2 3 ) .
Silica (SiO 2 )

16.22
68.92

1 .W IQ

18.24
65.56

Lime (CaO)

1 62

2 58

Magnesia (MgO)

Trace

Soda (NaaO)

1.56

2.08

Potash (KjO)

4 00

3 94

99.92 %

100.74%

PLATE 12

FIG. 1. Banded gneiss.

FIG. 2. Foliated gneiss.

IV. METAMORPHIC ROCKS

Before proceeding to describe in detail the metamorphic rocks,
it will be well to devote a brief space to a discussion of the
processes by which this metamorphism has been brought about.

The word metamorphism as used in geology includes changes
in the structure of rocks induced through agencies in part
physical, and in part chemical, in their nature. It is, in fact,
a very general terra, and indicates any transformation taking
place in the composition and structural features of rocks
of any kind, whether sedimentary or igneous, and from any
cause whatever. Bocks laid down in the form of sediments may
become so deeply buried as to be subject to intense heat from
the earth's interior, as well as to pressure from weight of
the overlying material. In this way, a partial or complete
fusion of the constituents takes place, which is followed by a
crystallization whereby the original fragmental nature may be
wholly or in part obscured. This form of change is included
under the general name of regional metamorphism. In this
manner, it was once assumed, were formed the gneisses, a part
of the granites, and the vast series of crystalline schists and
calcareous rocks (marbles, etc.). It has, however, been shown
that the banded and foliated structure shown by gneisses and
schists is not necessarily an indication of an original bedded
structure, but may be due to pressure acting throughout long
periods of time, accompanied by the heat thereby generated. A
common and readily understood illustration of this principle of
metamorphism by compression is offered by the roofing slates.
These, first laid down as fine silts, rarely show their eminent
cleavages whereby they are rendered so useful to man, parallel
to their original bedding, but inclined at any and all angles
thereto. In such cases the bedding is usually indicated by the
dark bands or "ribbons" which are so evident on a split surface.

But it is not alone the fragmental rocks which thus become
schistose under pressure. Originally massive, igneous rocks,
in regions of profound disturbances have been found converted
into schistose aggregates, indistinguishable from rocks ordinarily

135

136 METAMOEPHIC EOCKS

assumed to be sedimentary. The changes in these cases are rarely
purely physical, though the chemical alteration may be small.
The ultimate composition of a rock may remain essentially the
same, while the method of combination of its various elements
has undergone extensive alteration. Quartzes and feldspars
may be crushed and distorted, drawn out into lens-shaped and
variously elongated forms, while secondary minerals like feld-
spars, quartz, zoisite, garnet, hornblende, epidote, and the micas
are abundantly generated.

One of the commonest results of pressure effects upon igneous
rocks is the conversion of augite or other minerals of the pyrox-
ene group into hornblendes. The coarse hypersthene gabbro
occurring about Baltimore is found locally altered into a rock
consisting essentially of a schistose aggregate of hornblende and
plagioclase feldspars, or what, on mineralogical grounds, might
be classed as a diorite. 1 The chemical composition in this case
has undergone no appreciable change; there has been simply a
molecular rearrangement of the particles. In such cases proof
of the character of the change that has taken place is usually
found in the fractured and otherwise distorted condition of many
of the constituent minerals, as well as intermediate stages of
alteration, whereby a residual augite crystal is found enclosed
in an envelope of secondary hornblende, as shown in Fig. 1, on
p. 36. To the secondary minerals formed in this way the tech-
nical name paramorphic is applied. To such changes as are
above described the name dynamic metamorphism is given.

The protrusion of a mass of molten matter into the over-
lying strata may give rise to a series of changes differing from
the last in that they are due mainly to heat and to the chemical
action of accompanying vapors and solutions. Since these
changes are confined to limited areas along the line of the
contacts between the two bodies, they are defined as contact
met amor phisms.

A common form of metamorphism is manifested in the pro-
duction of a quartzite from siliceous sandstone. This, in its
simplest form, is brought about by a secondary deposit of silica
about the original rounded granules of sand, whereby the entire
mass is converted into an aggregate of quartz crystals, the out-
lines of which are more or less imperfect through mutual in-
terference in process of growth. The microscopic structure of

1 Bull. 28, IT. S. Geol. Survey, 1886.

CONTACT METAMOEPH1SM

137

a quartzite of this nature is shown in Fig. 13. In this case the
original rounded granules are readily recognized from the fact
that not merely did they
contain small cavities and
needle-like enclosures, but
exteriorly they were cov-
ered with a thin pellicle
of iron oxide, while the
secondary deposit, which
now fills all the inter-
spaces, is free from en-
closures of all kinds and
quite pellucid.

In many quartzites a
shearing force has acted
a prominent part, where-
by the granules have be-
come elongated and more
or less pulverized along Fl ?' 13.-Microstructure of quartzite, show-
. . , ing secondary deposit of silica about the

their margins by the original quartz grains,
friction of rubbing one

over the other. In such cases mica and other secondary min-
erals are often developed, and the rock passes over into a
mica schist.

Still another form of change, or metamorphism, is that
known by the name of metasomatosis, a process of indefinite
substitution and replacement. Through the chemical action
of percolating solutions certain constituents of a rock may be
leached out and replaced by others In indefinite proportions.
It is by such processes that have originated a large share of
the serpentinous rocks, dolomites, etc. The mineral olivine,
an anhydrous ferruginous silicate of magnesia, passes over into
serpentine by a simple process of hydration, and a more or less
complete change of its combined iron from the ferrous to the
ferric state. Provided there be no loss in silica, this change in the
olivine, according to T. Sterry Hunt, must be accompanied by an
increase of volume amounting to some 33%. Through the hy-
dration of eruptive olivine-bearing rocks, or rocks rich in other
magnesian silicate minerals, have originated a large proportion
of the so-called serpentines and verd-antique marbles. Many
serpentines and serpentinous limestones are derived from meta-

138 METAMOKPHIC KOCKS

morphic rocks rich in lime-magnesian pyroxenes or amphiboles,
as malacolite and tremolite. To such an origin are to be referred
the serpentinous limestones of Essex County, New York ; Easton,
Pennsylvania, and Montville, New Jersey. In the last-named
instance the original rock was coarsely crystalline dolomitic
limestone containing numerous nodular masses of white pyroxene
(malacolite). Under this metasomatic process the pyroxenes
yielded up their calcium, which recrystallized as calcite, while
the silica and magnesia, combined with some 13% of water, re-
mained as a beautiful green and yellow serpentine. The trans-
formation was accompanied by a considerable increase in bulk,
whereby the exterior of the nodules, pressed against the rough
walls of the enclosing rock, became scratched and polished like
boulders from the glacial drift, or the entire mass even took on a
platy, schistose structure. Figure 8, from a specimen in the
National Museum, illustrates a transitional phase of this change,
the interior rounded mass of a gray color being of still unaltered
pyroxene, while the dark material forming the exterior shell,
or traversing the gray in fine thread-like veins, is the secondary
serpentine. In a like manner in all probability originated the
peculiar structure imitative of animal organisms known as
Eozoon Canadense. 1

The conversion of a limestone into a dolomite is believed to
have been brought about by a somewhat similar process. Indeed
it is doubtful if this last-named rock is ever a product of direct
sedimentation or precipitation. Although sea-water contains
from three to four times as much magnesia as lime, evidence is
wanting to show that the material is ever secreted in appre-
ciable quantities by marine animals, and hence the sedimentary
deposits must be correspondingly lacking in this constituent. It
has been argued by Beaumont and others that through a process
of partial molecular replacement (metasomatosis) pre-existing
limestones were converted into dolomites, the process consisting
in the replacement of every other molecule of calcium carbonate
by one of the magnesium carbonate. As the dolomite molecule
is the more dense of the two, such replacement, in any given
limestone bed, must result in a contraction amounting to some

1 See On the Serpentine of Montville, New Jersey, Proc. U. S. National
Museum, Vol. XI, 1888; Notes on the Serpentinous Kocks of Essex County,
New York, etc., ibid., Vol. XII, 1889; and On the Ophiolite of Thurman,
Warren County, New York, Am. Jour, of Science, Vol. XXXVII, 1889.

METASOMATOSIS 139

12.5%. Assuming that a dolomitic mass resulting in this way
is of the same bulk as the original limestone, this shrinkage
must manifest itself in the production of interstitial rifts and
cavities, such as do actually occur in many dolomitic lime-
stones, as those of the Ohio Trenton formations. The principal
objection to this theory lies in the difficulty of accounting for
the large amount of magnesia in solution; whence its source,
etc. The same objections apparently apply to the explanation
given by M. C. Klement. 1 This writer describes a series of
experiments in which solutions of sodium chloride and magne-
sium sulphate were made to act upon pulverized calcite and
aragonite. From the results obtained, he concluded that dolo-
mite is formed by the action of sea-water, concentrated in en-
closed basins and heated by the sun, on the aragonite deposited
by marine organisms, in such a way that a mixture of carbon-
ates of calcium and of magnesium is first produced, which is
subsequently converted into dolomite.

Still another theory regards the dolomite as a residuary
product formed by the leaching out of the lime carbonate from
beds of impure, slightly magnesian limestone, leaving behind the
less soluble magnesian carbonate. The amount of material lost,
and the consequent contraction of the original beds, must neces-
sarily vary with their purity ; but in any case where the residual
mass has reached the condition of a true dolomite, the propor-
tional loss must have been enormous, since in no cases are un-
altered sediments known to contain more than 4 or 5% of mag-
nesian carbonate. This theory in its turn is apparently rendered
invalid by the presence in the dolomites of very perfect casts of
fossils which have undergone no crushing or distortion what-
ever, and which show that the beds as a whole, so far from
having undergone a shrinkage of 95% and upwards, are of es-
sentially the same bulk as when laid down. 2 The recent suggestion
of Professor J. W. Judd, on this point, seems in the present
state of knowledge most satisfactory. From an examination of
the deep borings obtained on the Atoll of Funafuti* he was led
to conclude that the pronounced dolomitization found in the
deeper lying rocks of the reef was due, as in the cases above

1 Bull. cle la Societe Geologique de Beige, Tome IX, 1895.
2 See The Magnesian Series of the Northwestern States, by C. W. Hall
and F. W. Sardeson. Bull. Geol. Soc. of America, Vol. VI, 1895, p. 167.
Eeport of Coral Eeef Committee, Eoyal Society of London, 1904, p. 387.

140 METAMOEPHIC EOCKS

mentioned, to a double decomposition and gradual replacement
of the calcium in the carbonate, by magnesium, the continual
percolation of sea-water with its normal content of magnesia
being sufficient to bring about the result. It was noted, however,
that in none of these cases did the per cent of magnesian car-
bonate quite reach that of true dolomite, 40 to 42% being the
maximum amount of this constituent found.

Yet another form of change in the structure and mineral
composition of a rock is that brought about through the action
of water below the zone of oxidation and of true weathering.
It may be best described as a process of hydro-metamorphism,
since the influence of water is paramount. It is to this form
of metamorphism that is due the production of secondary epidote,
chlorite, sericite, leucoxene and various zeolitic compounds from
pre-existing minerals without in any way changing the character
as a geological body of the rock mass in which they occur. Such
changes are in part metasomatic, and in many instances are
rendered more intense by dynamic causes. This form of change
has, unfortunately, been too frequently confounded with wea-
thering. 1

Under the head of metamorphic, then, is grouped a large
series of rocks which have been changed from their original
condition through the dynamical and chemical agencies above
described, and which may have been in part of aqueous and in
part of eruptive origin. Were it possible, it might have been
better to describe each class of these rocks together with the
corresponding igneous or aqueous form from which it was de-
rived by this process of change. In only too many cases, how-
ever, the metamorphism has been so complete as to quite obliter-
ate all such traces of the original character as would lead to safe
and satisfactory conclusions, and consistency demands that all
be grouped together.

Accordingly as they vary in structure the metamorphic rocks

i While it is true that no new compound can be formed without first a
breaking up, or decomposition, of those already existing, still, as this de-
composition affects only the individual minerals, and not the integrity of
the rock mass as a whole, it would seem preferable te include such changes
under the name of alteration and metamorphism. Weathering it certainly
is not, though it is essentially the form of change which Eoth (Allegemeine
u. Chemische Geologic, Vol. I, pp. 159-412) has designated as complex
weathering (Complicirte Verwitterung}. See also A Discussion of the Use
of the Terms Eockweathering, Serpentinization and Hydrometamorphism,
Geological Magazine, London, Aug., 1899, and American Geologist, Oct., 1899.

STEATIFIED OE BEDDED 141

may be divided into two general groups : 1. Stratified or bedded ;
2. foliated or schistose.

1. STRATIFIED OR BEDDED

(1) THE CEYSTALLINE LIMESTONES AND DOLOMITES

Here are included the metamorphosed form of the sedimentary
rocks described on p. 125.

Mineral Composition. The essential constituent of the crys-
talline limestones is the mineral calcite. The common acces-
sories are minerals of the mica, amphibole, or pyroxene group,
and frequently sphene, tourmaline, garnets, vesuvianite, apatite,
pyrite, graphite, etc.

Chemical Composition. As may be inferred from the mineral
composition, these rocks, when pure, consist only of calcium
carbonate. They are, however, rarely if ever found in a state
of absolute purity, but show more or less magnesia, alumina,
and other constituents of the accessory minerals. The analyses
given on p. 127 will serve equally well here, and need not be
repeated.

Structure. The limestones are eminently stratified rocks,
though this peculiarity is not always sufficiently marked to be
seen in the hand specimen. The purest and finest crystalline
varieties often show a granular texture like that of loaf sugar,
and hence are spoken of as saccharoidal limestones. Statuary
marble is a good illustration of this type. Under the micro-
scope the stone is shown to be made up of small grains, which,
having mutually interfered in process of growth, do not possess
perfect crystal outlines, but are rounded and irregular in out-
line, as shown in Fig. 14. All grades of textures are common,
the coarser forms sometimes showing individual crystals an inch
in length. Though in their unchanged conditions highly fossil-
iferous or tufaceous, these structural features may be wholly
or in part obliterated by crystallization.

Colors. The color of pure limestone is snow-white, as seen
in statuary marble. Other common colors are pink or reddish,
greenish, blue-gray through all shades of gray to black. The
pink and red colors are due to iron oxides, the greenish as a
rule to micaceous minerals, the blue-gray and black to carbon-
aceous matter.

142

METAMOKPHIC ROCKS

Geological Age and Mode of Occurrence. The crystalline

limestone and dolomites
are but the metamor-
phosed sedimentary de-
posits such as have al-
ready been described on
p. 125. They occur asso-
ciated with rocks of all
ages, but only in regions
that have been subjected
to disturbances such as
the folding and faulting
incident to mountain-
making, or the heat from
intruded igneous rocks.
From an economic stand-
point, the rocks of this
group are of great eco-

FIG. 14. Microstructure of crystalline lime-
stone (marble).

nomic value for structural and decorative purposes.

Classification and Nomenclature. It is common to speak of
this entire group of rocks as simply limestones, though many
varietal names are often rather indefinitely applied. The name
marble is given to any calcareous or magnesian rocks of such
quality as to be utilized in decorative work or high grade con-
struction. Argillaceous and siliceous limestones carry clayey
matter and sand. Dolomite (so named after the French geologist
Dolomieu) consists of 45.50% carbonate of magnesia and 54.40%
carbonate of lime, as already noted. The names ophiolite and
ophicalcite are popularly applied to stones consisting of a granu-
lar aggregate of calcite and serpentine, such as occur in Essex
County, New York, and are used as marbles.

2. FOLIATED OR SCHISTOSE

(1) THE GNEISSES

Gneiss, from the German Gneis, a term used by the miners
of Saxony to designate the country rock in which occur the
ore deposits of the Erzgebirge. The word is pronounced as
though spelled nice.

Mineral and Chemical Composition. The composition of the

THE GNEISSES 143

gneisses is essentially the same as that of the granites, from
which they differ only in structure and origin. They, how-
ever, present a greater variety and abundance of accessory
minerals, chief among which may be mentioned (besides those
of the mica, hornblende, or pyroxene group) garnet, tourmaline,
beryl, sphene, apatite, zircon, cordierite, pyrite, and graphite.
Structure. Structurally the gneisses are holocry stall ine gran-
ular rocks, as are the granites, but they differ in that the various

FIG. 15. Microstrutture of gneiss, showing at the point a broken feldspars.

constituents are arranged in approximately parallel bands or
layers, as shown in PI. 12.

In width and texture these bands vary indefinitely. It is
common to find bands of coarsely crystalline quartz several
inches in width, alternating with others of feldspar, or feld-
spar, quartz, and mica, or hornblende. A lenticular structure
is common, produced by lens-shaped aggregates of quartz or
feldspar, about and around which are bent the hornblendes or
mica laminae. The rocks vary from finely and evenly fissile
through all grades of coarseness, and become at time so mas-
sive as to be indistinguishable in the hand specimens from
granites.

Colors. Like the granites, the gneisses are all shades of gray,
greenish, pink, or red.

Geological Age and Mode of Occurrence. The true gneisses
are among the oldest crystalline rocks, and have been considered

144 METAMOEPHIC BOOKS

by many geologists as representing "portions of the primeval
crust of the globe, traces of the surface that first congealed upon
the molten nucleus." By others they are regarded as meta-
morphosed sedimentary deposits resulting from the breaking
down of still older rocks, and may not in themselves, therefore,
be confined to any particular geological horizon. They are in
large part, however, unquestionably the oldest known rocks,
lying beneath or being cut by all rocks of later formation or in-
jection.

The origin of the gneisses, as already suggested, is in many
cases somewhat obscure, the banded or foliated structure being
considered by some as representing the original bedding of the
sediments, the different bands representing layers of varying
composition. This structure is now however, considered to be
due to mechanical causes, and in no way dependent upon origi-
nal stratification. The name, as commonly used, is made to in-
clude rocks of widely different structure, which are beyond doubt
in part sedimentary and in part eruptive, but in all cases
altered from their original conditions.

This alteration, it should be stated, has been brought about
not by heat and crystallization alone, but in many cases by
processes of squeezing, crumpling, and folding so complex as
almost to warrant the application of the term kneading. It is
even possible to conceive that some of them may be original
massive or foliated rocks into which eruptive materials have
since been injected along lines of foliation or of weakness due to
shearing, and the entire mass again submitted to such a knead-
ing as to render it practically impossible to now decide what
are portions of the original rock and what of the subsequently
injected.

The close chemical relationship which may exist between
clastic, metamorphic, and eruptive rocks is shown in the selected
series of analyses given on the following page.

Figures 1 and 2 on PL 12 show two rather extreme types of
these gneissoid rocks. Figure 1 is that of a banded gneiss from
Madison County, Montana. In Fig. 2 is shown a foliated rather
than a banded rock, and whatever may have been its origin, it
undoubtedly owes its foliated structure to dynamic agencies.
The effect of the shearing force whereby the foliation was pro-
duced is evident in the figure to the left and just above the
centre, where an elongated feldspar is seen broken transversely

THE GNEISSES

145

TABLE ILLUSTRATING CHEMICAL SIMILARITY OP CLASTIC AND CRYSTALLINE

KOCKS.

CONSTIT CENTS

SHALK

iij

pgo

III

VII

Silica (Si0 2 ) . .

68.18

%
61.96

%
69.24

69.94

%
61.91

%
60.32

65.69

Titanium oxide (Ti02)

1 66

Not det

0.31

Alumina (A1 2 O 3 ) ....
Ferric oxide (Fe 2 O 3 ) . . .
Ferrous oxide (FeO)

16.20
4.10

19.73
4.60

14.85
2.62

13.15

2.48

21.73
4.73

23.10
7.05

15.23
4.39

Ferrous sulphide f FeS 2 )

4.33

Manganese oxide (MnO) . .
Lime (CaO)

1.75

Trace
0.35

0.45
2.10

0.70
3.08

0.09

....

Not det.
2.63

Magnesia (MgO)

0.48

1.81

0.96

Trace

0.59

0.87

2.64

Soda (Na 2 0)

2.88

0.79

4.30

5.43

0.25

0.49

2.12

Potash (K 2 O)

6.48

2.50

4.33

3.30

3.16

3.83

2.00

1.82

0.70

1.01

7.43

4.08

4.70

100.07

99.53

99-55

99.09

99.89

99.74

99.71

I. Granite: Syene, Egypt. II. Gneiss: St. Jean de Matha, Province of
Quebec, Canada. III. Gneiss: Trembling Mountain, Province of Quebec,
Canada. IV. Sandstone: Portland, Connecticut. V. Shale: England. VI.
Slate: Lancaster County, Pennsylvania. VII. Disintegrated granite: Dis-
trict of Columbia.

in four pieces. The same features are brought out even more
plainly in Fig. 15, on page 143, which shows the structure of this
same gneiss as seen under the microscope.

As in the present state of our knowledge it is in most cases
impossible to separate what may be true metamorphosed sedi-
mentary gneisses from those in which the foliated or banded
structure is in no way connected with bedding and which may
or may not be altered eruptives, all are grouped together here.

Classification and Nomenclature. The varietal distinctions
are based upon the character of the prevailing accessory min-
eral, as in the granites, forming a parallel series. We thus
have biotite gneiss, Muscovite gneiss, biotite-muscovite gneiss,
hornblende gneiss, etc.

The name granulite or leptynite is applied to a banded quartz-
feldspar rock, the constituents of which occur in the form of
11

146

METAMOEPHIC KOCKS

small grains and show under the microscope a mosaic structure.
The Saxon granulites are regarded by Lehman as eruptive
rocks altered by pressure. Halleflinta is a Swedish name for
a rock resembling in most respects the eruptive felsites or quartz
porphyries already described. Porphyroid is also a felsitic rock
with a more or less schistose structure, and with porphyritic
feldspar or quartzes.

Inasmuch as the structure characteristic of gneisses is found
developed in rocks of diverse types, many petrologists now use
the term in an almost wholly structural sense, as in itself non-
committal as to composition or origin, but merely designating a
rock of foliated or schistose structure. C. H. Gordon has pro-
posed 1 a scheme of classification of gneissoid rocks as below
which has much in its favor.

CLASSIFICATION or GNEISS

ANALOGOUS MASSIVE TYPE

OF IGNEOUS ORIGIN

OBIGIN UNKNOWN

Granite :
Biotite granite . . .
Hornblende granite . .

Syenite :
Hornblende syenite . .

Mica syenite ....
Pyroxene syenite . .
Diorite :
Micadiorite ....
Gabbro

Granite gneiss :
Biotite granite gneiss .
Hornblende granite \
gneiss . . . . /
Syenite gneiss :
Hornblende syenite )
gneiss . . . . /
Mica syenite gneiss. .
Pyroxene syenite gneiss
Diorite gneiss :
Mica diorite gneiss . .
Gabbro gneiss

Granitic gneiss :
Biotite granitic gneiss.
Hornblende granitic
gneiss.
Syenitic gneiss :
Hornblende syenitic
gneiss.
Mica syenitic gneiss.
Augite syenitic gneiss.
Dioritic gneiss :
Mica dioritic gneiss.
Gabbroic gneiss or gab-

Pyroxenite

Pyroxenite gneiss

brie gneiss.
Pyroxenitic gneiss

(2) THE CKYSTALLINE SCHISTS

Under this head are grouped a large and extremely variable
series of rocks, differing from the gneisses mainly in the lack of
feldspar as an essential constituent. They consist, therefore,
essentially of granular quartz, with one or more minerals of the
mica, chlorite, talc, amphibole, or pyroxene group. In acces-

Geol. Soc. of America, Vol. VII, p. 122.

GNEISS 147

sory minerals the schists are particularly rich. The more
common of these are feldspar, garnet, cyanite, staurolite,
tourmaline, epidote, rutile, magnetite, menaccanite, and pyrite.
Through an increase in the proportional amount of feldspar the
schists pass into the gneisses, and through a decrease in mica,
hornblende, or whatever may be the characterizing mineral,
into the quartz schists, in which quartz alone is the essential
constituent. Occasional forms are met with quite lacking
in quartz and other accessory minerals and consisting only of
schistose aggregates of minerals of a single species, as is the
case with the pyrophyllite schists (or, more properly, schistose
pyrophyllites) from North Carolina, talcose schists, and with
the more massive * ' soapstones. "

The rocks of this group are characterized as a whole by a
pronounced schistose structure, due to the parallel arrangement
of the various constituents, this structure being most pro-
nounced in those varieties in which mica is the predominating
accessory mineral. They are ordinarily considered as having
originated from the crystallization of sediments, and in many
cases the microscope still reveals existing " traces of the origi-
nal grains of quartz sand and other sedimentary particles of
which the rocks at first consisted. " Like the gneisses, they
are in part, however, mechanically deformed massive rocks and
their schistosity in no way relates to true bedding, as has been
already noted (p. 144).

The varietal names given are dependent mainly upon the
character of the prevailing ferro-magnesian silicate. We thus
have mica schists, chlorite schists, talc schists, hornblende, actinol-
ite, glaucophane schists, etc. The term slate was originally
applied to these and other types of rocks of schistose or fissile
character. In the arrangement here adopted this last term is
restricted to the argillaceous fragmental or semi-crystalline rocks
next to be described.

Of the above-mentioned varieties the mica schists are the
most common and widely distributed, the mica being in some
cases biotite, in others muscovite, or perhaps a mixture of the
two. The principal accessories sufficiently developed to be con-
spicuous are staurolites, chiastolites, garnets, and tourmalines.
In the sericite schists the hydrous mica sericite prevails; para-
gonite schist carries the hydrous sodium-mica paragonite; ot-
trelite schist carries the accessory mineral ottrelite.

148 METAMORPHIC ROCKS

The name phyllite is used by German petrographers to desig-
nate a micaceous semi-crystalline rock standing intermediate
between the true schists and clay slates. Quartzite is a more
or less schistose or banded rock consisting essentially of crys-
talline granules of quartz. Such originate from the induration
of siliceous sandstones as already explained.

The quartzites consist, as a rule, only of silica, or silica
colored brown and red by iron oxides. At times a greenish tinge
is imparted through the development of chloritic minerals; ac-
cessory minerals are not, as a rule, abundant.

The hornblende schists are, as a rule, less finely schistose than
are the mica-bearing varieties, owing to the fact that the mineral
hornblende itself has not a platy structure. The glaucophane
schists are perhaps the least abundant. Such have been described
from the Isle of Syra, in the Mediterranean Sea, Switzerland,
Wales, and Italy ; a more massive form, probably an altered erup-
tive, is found near the mouth of Sulphur Creek, Sonoma
County, and other parts of California. Amphibolite is the name
given to an extremely tough and often massive rock of obscure
origin, consisting essentially of the mineral amphibole or horn-
blende. In some instances actinolite and tremolite take the place
of the common hornblende. The tremolite rock may undergo
alteration into serpentine under proper conditions. Eclogite is
a tough, massive, or slightly schistose rock, consisting of a
grass-green variety of pyroxene, and small red garnets, with
which are frequently associated bluish kyanite, green hornblende
(smaragdite), and white mica. Garnet rock, or garnetite, is a
crystalline granular aggregate of garnets with black mica, horn-
blende, quartz, and magnetite. Kinzigkite is a somewhat similar,
though fine-grained and compact, rock consisting of garnets,
plagioclase feldspar, and black mica, which is found in Kinzig
and the Odenwald.

Many of the rocks of this group are but products of dynamic
or contact metamorphism ; this is the case with many of the
chiastolite and argillaceous schists or roofing slates. Eocks of
the latter group pass by insensible, gradations into clastic ar-
gillites. They owe their cleavable property to shearing, as
already explained. Under the microscope these rocks are
found to be quite variable. Hawes described clay slate from
Littleton, New Hampshire, as consisting of a mixture of quartz
and feldspar, in particles as fine as dust. They contained also

GNEISS

149

amorphous carbonaceous matter and little needles of a mineral
assumed to be mica. A slate from Hanover, in the same state,
contained garnets and staurolites. Wichman found slates from
Lake Superior to consist of a colorless, isotropic ground-mass
carrying quartz and feldspar particles, scales of iron oxide, car-
bonaceous matter, minute tourmalines, and mica fragments,
while T. Nelson Dale has described the roofing slate of western
Vermont and eastern New York as composed of quartz, a little
plagioclase feldspar, muscovite, chlorite, the carbonates of lime,
magnesia and iron, zircon, rutile and pyrite. The chemical com-
position of the slates is given on p. 119.

Chemical Composition. As may be readily imagined, the
schists vary indefinitely in composition. The table given below
is intended to show the composition of a few characteristic types
only. All gradations, from the most acid of quartzites to the
most basic of the amphibolites, may readily be found.

CHEMICAL COMPOSITION OF QUARTZITES AND SCHISTS

CONSTITUENTS

III

Silica (Si0 2 )

82.38 %

49.00 %

52.39%

49.18%

50.81 %

97.1%

Alumina (A1 2 8 ) . . .

11.84

23.65

16.33

15.09

4.53

1.39

Ferric oxide (Fe 2 3 ) . .

....

8.07

1.64

12.90

3.52

1.25

Ferrous oxide (FeO) . .

2.28

....

1.44

....

4.26

....

Lime (CaO)

....

0.63

8.76

10.59

....

0.18

Magnesia (MgO) . . .

1.00

0.94

4.70

5.22

31.55

0.13

Potash (K 2 O) ....

0.83

9.11

1.42

1.51

....

Soda (Na 2 O)

0.38

1.75

2.59

3.64

0.77

3.41

0.17

1.87

4.42

99.48%

96.56%

89.44%

100.00%

99.09%

100.05%

I. Mica schist: Monte Eosa, Switzerland. II. Sericite schist: Wisconsin.
III. Hornblende schist: Grand Eapids, Wisconsin. IV. Chlorite schist:
Klippe, Sweden. V. Talc schist : Gastein, Austria. VI. Quartzite : Chickies
Station, Pennsylvania. All analyses quoted from J. F. Kemp's Handbook
of Eocks, 1904.

PART III

THE WEATHERING- OP ROCKS

" In the economy of the world, I can find no traces of a beginning, no
prospect of an end. ' ' HUTTON.

THE stability of chemical compounds is governed by prevail-
ing conditions. A form of combination stable under conditions
existing to-day may, under those of to-morrow, become impos-
sible. As was suggested in the introductory chapter, the con-
ditions under which the more superficial portions of the earth's
crust exist are ever changing, and as a result old compounds
are broken up and new continually formed. All over the earth
rocks laid down as sediments on oceanic floors have been up-
lifted, folded, faulted, and pushed out of place until brought
under influences as different from those under which they were
formed as it is possible to conceive. Molten magmas cooling sud-
denly on the immediate surface formed compounds in which mere
loss of heat was the controlling factor, but which time proves to
be unstable. Slow cooling, deep-seated magmas have been, and
are being, continually exposed by denudation, and thus brought
under new influences and environments. Hence a constant re-
adjustment is everywhere going on, which, as will be seen, is
manifold in its physical manifestations. As where an entire
building is razed to the ground, and another of quite different
architectural features constructed from the old materials; or
again, where, without change of general plan, old timbers are
here and there replaced by new, so here we have at work a
series of processes in part seemingly destructive and in part
constructive, but all tending toward one end.

The firm and everlasting hills we must learn to regard as
neither firm nor everlasting. Whole mountain chains of the
geological yesterday have disappeared from view, and as with
the ancient cities of the East, we read their histories only in
their ruins. Yet, in all this seemingly destructive process of
breaking down, decomposition, and erosion, there is traceable

150

PLATE 13

FIG. 1. Glaciated and exfoliated granite, near Cathedral Lake in the Sierra Nevada. U. S. G. S.
FIG. 2. Weathered biotite granite, near Morrison Creek, Yosemite, Calif. U. S. G. S.

* THf

UNIVERSITY

THE WEATHERING OF EOCKS 151

the one underlying principle of transformation from the un-
stable toward that which is to-day more stable. Nothing is
lost or wasted: It is a change which began with the beginning
of matter; which will end only with the blotting out of matter
itself. There are no traces of a beginning, there is no prospect
of an end.

I. THE PRINCIPLES INVOLVED IN ROCK-
WEATHERING

The processes involved in this readjustment from unstable
to stable compounds, as above outlined, and of incidental soil
formation, are in part physical and in part chemical in their
nature ; they operate under every-varying conditions, and
through processes at times simple, or again complex. What
these processes are, and how they operate, it must be our purpose
to now consider.

It may be said at the outset, that whatever the forces en-
gaged, they are, with a few isolated exceptions, superficial,
they work from without downwards. However much they may
have accomplished since the first rock masses appeared above
the primeval ocean, in no case can the actual amount of debris
in situ have formed at one time more than a scarcely appreciable
film, geologically speaking, over the underlying and unchanged
material. The decomposing forces early lose their active prin-
ciples and become quite inert at depths comparatively insignifi-
cant. It is only where through erosion the results of the disinte-
gration are gradually removed, that the processes have gone on
to such an extent as to perhaps quite obliterate thousands of feet
of strata or of massive rock, and furnished the necessary debris
for the vast thicknesses of sandstone, shale, and slate which
characterize the more modern horizons. In certain isolated cases,
it is true, ascending steam and heated waters, arising from depths
unknown, have been instrumental in promoting decomposition,
as is well illustrated in the areas of decomposed rhyolites in the
Yellowstone National Park. Nevertheless, it is to the slow process
of superficial weathering that we owe a very large share of the
apparent rock decomposition and incidental soil formation. 1

1 The term weathering, as here used, is applied only to those superficial
changes in a rock mass brought about through atmospheric agencies, and
resulting in a more or less complete destruction of the rock as a geological

152 THE PRINCIPLES INVOLVED IN ROCK- WEATHERING

This transformation, as already noted, takes place through
processes that may be simple, or again complex. It is but
rarely that one, alone, prevails for any length of time, and as a
rule several or many go on together. Were it possible, it might
be well to consider briefly each of these in its turn and by itself.
From the fact, however, as above stated, that any one, either
physical or chemical, rarely goes on alone, it is thought best to
treat the subject as below, and describe in more or less detail
the action, first, of the atmosphere, second, of water, in both
the solid and liquid form, and third, that of plant and animal
life, finally considering the combined action of all these forces,
as manifested on the various types of rock which go to make up
the earth's crust.

So striking a phenomenon as the breaking down, or degenera-
tion as we may call it, of a mass of firm rock, naturally did not
escape the observation of the earlier workers in this and allied
branches of science, and the older literature from the time of
Hutton contains numerous references to it, though the full sig-
nificance of atmospheric agencies in bringing about the results,
seems not at first to have been fully realized.

The exciting cause of the degeneration, particularly in warm
latitudes, where phenomena of this nature are often more ap-
parent, has been a matter of some speculation, and at the out-
set it may be well to indicate in brief their tendencies.

Fournet, as quoted elsewhere, writing as early as 1833, in-
sisted upon the efficacy of water containing carbonic acid in

body, as where granitic rocks are resolved into sand and kaolinic material,
with liberation of carbonates of the alkalies and of lime, and oxides of
iron. It does not include those deeper-seated changes changes taking place
below the zone of oxidation which result mainly in hydration and the pro-
duction of new mineral species, as chlorite, sericite, zeolites, etc., but during
which the rock mass as a whole retains its individuality and geological
identity. The distinction is not one that has been sharply insisted upon,
and indeed geologists and petrologists as a rule have been extremely care-
less in their use of such terms as alteration, decomposition, and weathering.
For reasons above stated and others given on p. 140, it seems best to limit
the terms weathering and decomposition to processes involving the destruc-
tion of the rock mass as a geological body, and to designate the purely
mineralogical deeper-seated changes as alteration, which may or may not
be due wholly to hydrometamorphism. This is the distinction also made
by Van Hise in his work on metamorphism, though expressed somewhat
differently, his "belt of weathering " being that portion of the zone of
katamorphism extending from the surface down to the level of the ground
water.

OPINIONS OF EAKLY WORKERS 153

promoting the decomposition of igneous rocks, while Brongniart,
writing with particular reference to feldspathic decomposition
and the origin of kaolin, laid great stress on the acceleration
of the ordinary process of decay through the electric currents
resulting from the contact of heterogeneous rock masses. Dar-
win 1 believed the extensive decomposition observed by him in
Brazil, to have taken place under the sea, and before the present
valleys were excavated. Hartt 2 gave it as his opinion that the
decomposition was due to the action of warm rain water soaking
through the rock, and carrying with it carbonic acid derived
not only from the air, but from the vegetation decaying in the
soil as well, together with organic acids, nitrate of ammonium,
etc. Further, that the decomposition had gone on only in re-
gions once covered by forests. Heusser and Claraz 3 suggested
that the decomposition was brought about through the influence
of nitric acid. "It is without doubt determined by the violence
and frequency of the tropical rains, and by the dissolving action
of water, which increases with the temperature. It is necessary
to observe, moreover, that this water contains some nitric acid,
on account of the thunder storms which follow each other with
great regularity during many months of the year."

Belt, 4 in discussing the extensive decompositiori^observed by
him in Nicaragua, wrote: "This decomposition of the rocks
near the surface prevails in many parts >0f tropical America,
and is principally, if not always, confined to the forest regions.
It has been ascribed, and probably with reason, to the percola-
tion through the rocks of rain water charged with a little acid
from the decomposing vegetation." Hunt 5 thought the great
amount of decomposition observed by him in the Blue Ridge of
Virginia was a matter of great geological antiquity, and effected
at a time when a highly carbonate atmosphere and climate quite
different from the present prevailed.

,The elder Agassiz laid much stress on the decomposing effects
of the hot water from rainfall, 6 while Mills and Branner, 7 in

1 Geological Observations, p. 417.
2 Phys. Geog. and Geol. of Brazil.
8 Ann. des Mines, 5th series, Vol. 17, 1860, p. 291.
4 The Naturalist in Nicaragua, 1874.
B Proc. Boston Soc. Nat. Hist., Vol. 16, 1873.
'Journey in Brazil, p. 89.

7 Bull. Geol. Soc. of America, Vol. VII, 1896, also Journal of Geology,
Vol. VIII, 1900.

154 THE PRINCIPLES INVOLVED IN EOCK- WEATHERING

addition, attributed a part of the decomposition to the action
of decomposing organic matter carried into the ground by ants,
and also to the acid secretions of the ants themselves.

The chemical changes involved in the process of decompo-
sition received attention from several of the earlier workers,
among whom the names of Berthier, Forschamnler, Brongniart,
Gustav Bischof, and Ebelmen stand out in greater prominence.
More recently the name of Sterry Hunt becomes conspicuous,
while the purely geological side of the question has been ably
set forth in numerous papers by L. Agassiz, R. Pumpelly, N. S.
Shaler, 0. A. Derby, R. Irving, J. C. Branner, and others, to
which reference is frequently made in these pages.

1. ACTION OF THE ATMOSPHERE

Atmospheric air consists in its normal state of a mechanical
admixture of free nitrogen and oxygen in the proportion of
four volumes of the former to one of the latter. In addition are
small and comparatively insignificant amounts of various com-
bined gases and salts, of which carbonic acid is by far the most
abundant, constant, and, from the present standpoint, important.
Still smaller quantities of ammoniacal vapors exist, and in vol-
canic regions there have been detected appreciable but variable
quantities of sulphuric and hydrochloric acids as well. With
rare exceptions these last exist in combination as sulphates, chlo-
rides, and nitrates and with the exception of the last-named need
little consideration.

(1) Nitrogen, Nitric Acid, and Ammonia. Nitrogen, by it-
self, is believed to be wholly inoperative in promoting rock
decomposition. In works on agricultural chemistry, much has,
however, been written concerning the presence in the atmosphere
of the compounds of nitrogen, nitric acid, and ammonia, and it
will be well to devote a little space to a consideration of the
facts as known, and their possible application to the subject
under consideration.

The well-known experiments of Cloez, Boussingault, De Luca,
Kletzinsky, and Way, as well as the more recent ones of G. H.
Failyer, 1 prove conclusively the existence of ammonia and rarely
of nitric acid in the air, from whence they are brought to the
surface of the earth in the water of rainfalls.

1 Ammonia and Nitric Acid in Atmospheric Waters, 2d Ann. Rep. Kansas
Experiment Station, 1889.

ACTION OF THE ATMOSPHEEE

155

In nearly every case, however, the percentage of ammonia, as
determined, equalled or exceeded the amount necessary to com-
bine with the acid, forming thus ammonium nitrate. Failyer's
researches in Kansas, carried on for a period of four years,
during which time water was collected from 266 rainfalls,
showed in but seven instances nitric acid equalling or ex-
ceeding the ammonia. In all other reported cases the amount
is less, with the possible exception of a fall of hail at Nismes, in
1845, which is stated to have been sufficiently acid to be sour
to the taste. As direct promoters of rock decomposition, neither
atmospheric nitrogen nor free nitric acid need, then, very seri-
ous attention. The following tables are, however, of interest,
the first being abridged from Johnson's How Crops Feed, and
the second from Professor Failyer's paper above quoted.

AMOUNTS OF RAIN AND OF AMMONIA, NITRIC ACID, AND TOTAL NITROGEN
THEREIN, COLLECTED AT EOTHAMSTEDD, ENGLAND, IN THE YEARS 1855-56,
CALCULATED PER ACRE, ACCORDING TO MESSRS. LAWES, GILBERT, AND WAY.

Total . .

Quantity of rain in
Imperial gallons.
1 gal. = 10 Ib. water

Ammonia
(in pounds)

Nitric Acid
(in pounds)

Total Nitrogen
(in pounds)

1855

663.332

1856

616.051

1855

7.11

1856

9.53

1855

2.98

1856

2.80

1855

6.63

1856

8.31

AMOUNTS OF RAIN AND OF AMMONIA, NITRIC ACID, AND NITROGEN THEREIN,
COLLECTED AT MANHATTAN, KANSAS, 1887-90, ACCORDING TO G. H.
FAILYER.

Total Nitrogen.
Means for 4

Nitrogen in
ammonia.
Means for 3

Nitrogen in
nitric acid.
Means for 3

years

Parts per million of water ....

0.522

.388

0.156

Grammes per acre

1563.0

1196.0

480.0

Pounds per acre .

3.44

2.63

1.06

It has been demonstrated, however, that nitrogen compounds
and nitrogenous matter in the soil may become subject to nitri-
fication through the action of bacteria, whereby ammonia,
nitrous or nitric acid, carbon dioxide, and water are formed,
though, as Wiley says, " The ammonia and nitrous acid may

156 THE PEINCIPLES INVOLVED IN BOCK- WEATHERING

not appear in the soils, as the nitric organism attacks the latter
at once and converts it into nitric acid. " * ( See further under
influence of plant and animal life, p. 180.)

In considering the possible efficacy of these compounds, one
must not lose sight of the fact that the amount of nitrogen in
the soils is as a rule far too small to supply the demands of
growing plants, and it is probable that a very large proportion
of that which finds its way there is quickly taken up again by
these organisms. It is possible that other salts of ammonium
than the nitrate may be locally efficacious. M. Beyer, as quoted
by Van Den Broeck, 2 has shown that the feldspars decompose
very rapidly under the influence of water containing ammonium
sulphate or even sodium chloride, either of which substance may
be found in vegetable soil. Daubree, who experimented by means
of revolving iron cylinders, found, however, that the presence of
sodium chloride retarded decomposition. (See p. 174.)

(2) Carbonic Acid. The amount of carbonic acid in the air
under natural conditions is not a widely variable quantity, ex-
cepting near volcanoes and in the immediate vicinity of gaseous
springs. In the vicinity of large cities and manufactories
consuming great quantities of coal, the amount is increased.
Although carbonic acid is the most abundant gas given off by
decomposing vegetable matter, it has apparently been definitely
ascertained that the amount in regions of abundant vegetation
is no greater than elsewhere. This has been accounted for on
the assumption that, as fast as liberated, it is taken up by grow-
ing organisms or carried by rains into the soil. 3

1 Wiley, Principles and Practice of Agricultural Analysis.

2 Mem. sur les phenomenes d 'Alteration des Depots Superficial, p. 16.

3 The researches of Boussingault and Lewey (Mem. de Chemie Agricole,
etc.), as quoted by Johnson (How Crops Feed, p. 139), showed the following
proportions existing between the CO 2 of atmospheric air and that of various
soils:

CO 2 IN 10,000 PARTS
BY WEIGHT

Ordinary atmosphere 6 parts

Air from sandy subsoil of forest 38 parts

Air from loamy subsoil of forest 124 parts

Air from surface soil of forest 130 parts

Air from surface soil of vineyard 146 parts

Air from pasture soil 270 parts

Air from soil rich in humus 543 parts

ACTION OF THE ATMOSPHERE

157

Twenty-one tests of the air in various parts of Boston, during
the spring, 1870, showed the presence of 3.85 parts of carbonic
acid in 10,000. Eleven tests of the winter air in Cambridge
yielded 3.37 parts in 10,000.* Dr. J. H. Kidder found the out-
door air of Washington to contain 3.87 to 4.48 parts in 10,000,
while Dr. Angus Smith, after an elaborate series of experiments,
reported the atmosphere of Manchester (England) as contain-
ing 4.42 parts in 10,000. 2

These amounts are considerably in excess of those reported
by Miintz and Aubin, 3 who give the following figures relative
to the proportional amounts in 10,000, as determined at the
various widely separated stations. The amount, it will be per-
ceived, is slightly greater during the night than during the day.

CARBONIC ACID IN THE ATMOSPHERE

LOCALITY

DAT

NIGHT

Hayti

2704

2920

Florida

2897

2 947

Martinique ....

2735

2850

Mexico ....

2665

2860

Santa Cruz, Patagonia

2 664

2670

Chubut, Patagonia

2790

3120

Chili

2.665

2820

The general mean is then 2.78 parts in 10,000, that for the
night alone being 2.82. For the north of France the mean is
given as 2.962, for the plain of Vincennes 2.84, and for the
summit of the Pic du Midi 2.86.

Fischer, as quoted by Branner, 4 has shown that in rain and
snow water the amount of carbonic acid varies between 0.22%
and 0.45% by volume of water. Assuming that the mean of
these figures fairly represents the general average, it is easy,
knowing the rainfall of any region, to calculate the amount of
the gas thus annually brought to the surface. Professor Bran-
ner has thus calculated that from 3.21 to 11.80 millimetres of
carbonic acid (C0 2 ) are annually brought to the surface in cer-
tain parts of Brazil. The same method of calculation applied

1 2d Ann. Eep. Mass. State Board of Health, 1871.

2 Air and Eain, p. 52.

"Comptes Eendus, Vol. XCIII, 1881, p. 797; also XCVI, 1883, pp.
1793-97.
4 Op. cit.

158 THE PEINCIPLES INVOLVED IN KOCK-WEATHEBING

to the various parts of the United States, would give us for the
Atlantic coast states 3.75 mm. ; for the upper Mississippi val-
ley, 2.50 mm. ; for the lower Mississippi valley, 4.50 mm. ; and
for the northern Pacific states 6.25 mm. As it is mainly when
this carbonic acid is thus brought to the surface by the rain
and snows that its effects become of direct significance in the
present work, the matter may be dropped here, to be taken up
again when considering the chemical action of water.

(3) Oxygen. Under ordinary conditions oxygen is the most
active principle in atmospheric air, and to it is due the process
of oxidation which almost invariably characterizes the decom-
position of silicates and other minerals containing iron in the
protoxide state. Such oxidation is, however, almost inactive
unless aided by moisture, and a further discussion of the subject
may well be deferred, to be taken up again when discussing the
action of water.

(4) Heat and Cold. The ordinarily feeble action of the air
is greatly augmented through natural temperature variations.
That heat expands and cold contracts is a fact too well known
to need elaboration. That, however, the constant expansion
and contraction due to diurnal temperature variations may be
productive of weakness and ultimate disintegration in so inert
a body as stone, seems not so generally understood, or is, at
least, less well appreciated; hence a little space is devoted to
the subject here. Rocks, it must be remembered, as the writer
has noted elsewhere, 1 are complex mineral aggregates of low
conducting power, each individual constituent of which possesses
its own ratio of expansion, or contraction, as the case may be.
In crystalline rocks these various constituents are practically
in co-ntact. In clastic rocks they are, on the other hand, sepa-
rated from one another by the interposition of a thin layer of
calcareous, ferruginous, or siliceous matter which serves as a
cement. As temperatures rise, each and every constituent ex-
pands and crowds against its neighbor; as temperatures fall, a
corresponding contraction takes place. Since in but few regions
are surface temperatures constant for any great period of time,
it will be readily perceived that almost the world over there
must be continuous movement within the superficial portions of
the mass of a rock.

The actual amount of expansion and contraction of stone
1 Stones for Building and Decoration, Wiley & Sons, New York.

ACTION OF THE ATMOSPHEEE 159

urfder ordinary temperatures has been a matter of experiment
W. H. Bartlett 1 has shown that the average rate of expansion
for granite amounts to .000004825 inch per foot for each de-
gree Fahrenheit ; for marble .000005668 inch, and for sandstone
.000009532 inch. Adie, in a series of similar experiments, found
the rate of expansion for granite to be .00000438, and for white
marble .00000613 inch. 2 Slight as these movements may seem,
they are sufficient to in time produce a decided weakening and
afford a starting-point for other physical and chemical agencies
The writer well remembers the peculiar impressions produced
during one of his earlier trips into the comparatively arid
regions of Montana, at finding the slopes and valley bottoms
strewn with small, beautifully fresh, concave and convex
chips of a dense, coal-black, andesitic rock that occupied
the crest of one of the higher hills. So fresh were the frac-
tures, so free were they from oxidation or other signs of de-
composition, it was at first felt that they must be of human
origin, that they were chips flaked off by aboriginal workmen in
making stone implements, and some time was wasted in seeking
for the more complete results of their handiwork. It, however,
did not take long to convince him that the flakes were far too
abundant and too widely spread to have originated in any such
manner, while the finding, on the top of the hill, of the coal-
black rock, broken into larger columnar blocks, each with its
angles rendered more obtuse or even fluted by the springing off
of just such flakes, this, coupled with the knowledge that
during the day, exposed under a cloudless sky, the rocks became
so highly heated as to be uncomfortable to the touch, whilst at
night the temperature sank nearly to the freezing-point, sufficed
to teach, as it must have taught the most obtuse, that the ordi-
nary daily temperature variations were amply sufficient to ac-
count for the phenomenon. In cold climates, and particularly
where glaciation has prevailed, the results of such flaking are
sometimes strikingly manifest.- Explorers in Northern Labrador
brought back to the National Museum sheets of coarse red granite
of remarkably uniform thickness (about 5 mm.) found over areas
of many acres, still perfectly fresh and showing glacial striae or
scouring on their upper surfaces. G. K. Gilbert, of the United

1 Am. Jour, of Science, Vol. XXII, 1832, p. 136.

2 Trans. Koyal Soc. of Edinburgh, Vol. XIII, p. 366.

160 THE PRINCIPLES INVOLVED IN ROCK-WEATHERING

States Geological Survey, has photographed quite similar phe-
nomena in the high Sierras of California. (See Fig. 1, PI. 13.)

Shaler states 1 that rock surfaces in the eastern United States
may be subjected to temperature varying from 150 F. at
midday in summer to and below in winter. This change of
150 in a sheet of granite 100 feet in diameter would produce a
lateral expansion of about one inch of surface. That this ex-
pansion must tend to lessen the cohesion and tear the upper
from the deeper lying layers, is self-evident. As exemplifying
this, Professor Shaler states that there are on Cape Ann (Massa-
chusetts) hundreds of acres of bare rock surface completely
covered with blocks of stone, which have been separated from
the mass beneath by just this process. 2

The size of such flakes may vary from those of microscopic
proportions to masses of several tons' weight. The higher
slopes of Lone Mountain, east of the Madison, in Montana, are
covered above timber line with thousands upon thousands of loose
flakes of all sizes up to ten or more feet in diameter. Such, here,
as in general, are characterized by a roughly lenticular outline
in cross-section, possessing a large superficial area in proportion
to their thickness, and are further distinguished from boulders
of decomposition by the entire freshness of their materials even
to the very surface. In close-grained, black andesitic and basaltic
rocks the chip or flake not infrequently shows a beautiful concave
and convex form and is greatly elongated in proportion to its
breadth, resembling the long and slender chips of obsidian or
flint found on the sites of aboriginal workshops. The surface
left by the springing off of the flakes is fluted as though the work
were done with a carpenter 's gouge.

In regions of great extremes of daily temperature the rup-

1 Proc. Boston Soc. of Nat. History, XII, 1869, p. 292.

2 The rifting action of heat upon granitic masses is said to have been
made a matter of quarry utility in India. It is stated (Nature, January 17,
1895) that a wood fire built upon the surface of the granite ledge and
pushed slowly forward causes the stone to rift out in sheets six inches or
so in thickness, and of almost any desired superficial area. Slabs 60 X 40
feet in area, varying not more than half an inch from a uniform thickness
throughout, have been thus obtained. In one instance mentioned, the sur-
face passed over by the line of fire was 460 feet, setting free an area of
stone of 740 square feet of an average thickness of five inches. This stone
is undoubtedly one of remarkably easy rift, but the case will, nevertheless,
serve our present purposes of illustration.

ACTION OF THE ATMOSPHERE 161

turing of the masses from the parent ledge is sometimes attended
with gun-like reports sufficiently loud to be heard at a consid-
erable distance. H. von Streeruwitz states 1 that the rocks of the
Trans Pecos (Texas) region undergo a very rapid disintegration
from diurnal temperature variations, which here amount to
from 60 to 75 Fahr. "I frequently observed on the heights of
the Quitman Mountains a peculiar crackling noise and occasion-
ally loud reports, . . . and careful research revealed the fact
that the crackling was caused by the gradual disintegration and
separation of scales from the surface of the rock, and the loud
reports by crackling and splitting of huge boulders." The
scales thus split off vary in thickness from one-half to four
inches, and their superficial area from a few square inches to
many feet. This form of disintegration is necessarily confined
to slopes unprotected by vegetation, and is the more pronounced
the greater the diurnal variations.

In Arabia Petrea, according to Marsh, 1 "when a wind pow-
erful enough to scour down below the ordinary surface of the
desert and lay bare a fresh bed of stones is followed by a sudden
burst of sunshine, the dark agate pebbles are often cracked and
broken by the heat." According to Livingstone, the rock tem-
peratures in certain parts of Africa, on the immediate surface,
rise during the day as high as 137 F. and at night fall so rap-
idly as to throw off by their contraction sharp, angular masses
in sizes up to 200 pounds' weight. Stanley, in his reports,
is inclined to lay considerable stress on the effects of cold rains
upon the heated rock surfaces, though it is doubtful if this is
as powerful an agent as his descriptions would give us to under-
stand. (See further under action of water.) Throughout the
desert regions of lower California, as observed by the writer,
the granitic and basic eruptive rocks subject to very little
rainfall, and hence almost completely bare of vegetation, under
the blistering heat of the desert sun have weathered down into
dome-shaped masses, their debris in the form of angular bits of
gravel being strewn over the plain. Particles of this gravel,
when compared with those which are a product of chemical
agencies, are found to differ in that each, however friable, is a
complex molecule of quartz, feldspar and mica or other mineral
that may have composed the rock from which it was derived.

i4th Ann. Rep. Geol. Survey of Texas, 1892, p. 144.
2 The Earth as modified by Human Action, p. 552.
12

162 THE PRINCIPLES INVOLVED IN ROCK-WEATHERING

Aside from a whitening of the feldspathic constituent, due to
the reflection of the light from its parted cleavage planes,
scarcely any change has taken place, and indeed it more resem-
bles the finely comminuted material from a rock-crusher than
a product of natural agencies.

Owing, however, to the low conducting power of rocks, dis-
integration from this cause alone can go on to any extent only
at the immediate surface, and on flat and level plains, where
the debris is allowed to accumulate, must in time completely
cease. 1 It is only on hillsides and slopes, or where by the
erosive action of running water, or by wind, the debris is re-
moved, that such can have any geological significance, although
the rate of such disintegration is sufficiently rapid in exposed
places to be of serious consequence in stone used for architectural
application. (See further on p. 177, Action of Ice.)

1 Observations on soil temperatures made at the Orono, Maine, Experi-
ment Station showed that the mean daily range of temperatures from
April to October, at a depth below the surface of 1 inch, was 5.62; at a
depth of 3 inches, 5.26; at 6 inches, 1.9; and at 9 inches, 1.18; and at
12 inches very slight. At the depth of 1 inch the temperatre was lower
than that of the air by 2.4 ; at 3 inches by 2.11 ; at 6 inches by 3.16 ; at
9 inches by 3.94 ; at 12 inches by 4.18 ; at 24 inches by 5.78 ; and at 36
inches by 7.10.

The following table, compiled by Forbes (Trans. Royal Society of Edin-
burgh, Vol. XVI, 1849), from observations made near Edinburgh, Scotland,
during 1841-42, shows the range of earth temperatures at varying depths
in soil, sandstone, and trap rock.

DEPTH

TRAP ROCK

SAND OF GARDEN

CRAIGLEITU SANDSTONE

Max.

Min.

Range

Max.

Min.

Range

Max.

Min.

Range

3 feet . . .

52.85

38.88

13.97

54.50

37.85^

17.65

53.15

38.25

14.90

6 feet . . .

51.07

40.78

10.29

52.95

39.55

13.40

51.90

38.95

12.95

12 feet . .

49.00

44.20

4.80

50.40

43.50

6.90

50.30

41.60

8.70

24 feet . .

47.50

46.12

1.38

48.10

46.10

2.00

48.25

44.35

3.90

It has been shown that the thermal conductivity of rocks varies in direc-
tion according to their structure, being greatest in the direction of their
schistosity, where such exists. In massive, homogeneous rocks the con-
ductivity is the same in all directions. In finely fissile rocks, on the other
hand, it may be four times as great in the direction of their fissility as at
right angles thereto.

ACTION OF THE ATMOSPHERE 163

(5) Wind. But it is to the action of the air when in motion
to the wind that is due a very considerable part of atmos-
pheric work. Particles of sand drifting along before the wind
become themselves agents of abrasion, filing away on every hard
object with which they come in contact. As a matter of course,
this phenomenon is most strikingly active in the arid regions,
though the results, when looked for, are by no means wanting
in the humid east It is thought by Professor Egleston that
many of the tombstones in the older churchyards of New York
City have become illegible by the wearing action of the dust
and sand blown against them from the street. There is among
the heterogeneous collections of the National Museum at Wash-
ington a large sheet of plate glass, once a window in a light-
house on Cape Cod. During a severe storm, of not above forty-
eight hours' duration, this became on its exposed surface so
ground from the impact of grains of sand blown against it as to
be no longer transparent, and to necessitate its removal. Win-
dow panes in the dwelling-houses of the vicinity are, it is stated,
not infrequently drilled quite through by the same means.

Apply now this agency to a geological field in a dry region.
The wind, sweeping across a country bare of verdure and
parched by drought, catches up the loose particles of dust and
sand and drives them violently into the air in clouds, or sweeps
them along more quietly close to the surface, where they are
at first scarcely noticeable. The impact of a single one of these
moving grains on any object with which it may come in con-
tact is far too small to be appreciable; but the impact of
millions, acting through days, weeks, and years, produces re-
sults not merely noticeable, but strikingly conspicuous. We
have here, in fact, a natural sand blast, an illustration on a
grand scale of a principle in common use in glass-cutting, and
to a small extent in stone-cutting also. Constantly filing away
on every object with which they come in contact, the grains
go sweeping on, undermining cliffs, scouring down mountain
passes, wearing away the loose boulders, and smoothing out all
inequalities. Naturally the abrading action on exposed blocks
of stone is most rapid near the ground, as here the flying sand
grains are thickest. First the sharp angles and corners are
worn away, and the masses gradually become pear-shaped,
standing on their smaller ends. Finally the base becomes
too small for support, the stone topples over, and the process

164 THE PEINCIPLES INVOLVED IN EOCK-WEATHEEING

begins anew without a moment's intercession, and continues
until the entire mass disappears, becomes itself converted
into loose sand drifted by the wind and an agent for destruc-
tion. Professor W. P. Blake was the first, I believe, to call pub-
lic attention to this phe-
nomenon, having observed
it while in the Pass of
San Bernardino ( Cali-
fornia) in 1853. G. K.
Gilbert has also published
some interesting facts as
noted by himself while
geologist of the Wheeler
Expedition west of the
100th meridian, in 1878. 1
In acting on the hard
rocks, the sand cuts so
slowly as at times to pro-
duce only grooved or fan-
tastically carved surfaces,
often with a very high
polish. The geologists of the 40th Parallel Survey in 1878 de-
scribed like interesting phenomena as observed on the western
faces of conglomerate boulders exposed to the sand blast of the
desert regions of Nevada. The surface of the otherwise light-
colored rock was found to have assumed a dark lead-gray hue and
a polish equal to that of glass, while the sand had drilled irregular
holes and grooves, often three-fourths of an inch deep and not more
than an eighth of an inch in diameter, through pebbles and matrix
alike. Professors W. M. Davis, 2 G. H. Stone, 3 and J. B. Wood-
ward 4 have described pebbles occurring in the glacial deposits

1 It should be noted that the ' ' sand-blast carving ' ' described by Gilbert in
this report is not due wholly to the action of wind-blown sand. The rock
is fine calcareous shale. Through the solvent action of meteoric water the
calcareous cement is removed, the fine, argillaceous interstitial material
mechanically eroded, while the more resisting granules of quartz sand stand
in relief, giving rise to elevated points and ridges.

*Proc. Boston Soc. of Natural History, Vol. XXVI, 1893, p. 166.

3 Am. Jour. Science, Vol. XXXI, 1886, p. 133.

* Ibid., Jan., 1894, p. 63.

FIG. 16. Eock undermined by wind-blown
sand.

CHEMICAL ACTION OF WATER 165

of Massachusetts and of Maine, carved and facetted by the same
agencies. 1

2. CHEMICAL ACTION OF WATER

Pure water, though an almost universal solvent, nevertheless
acts with such slowness upon the ordinary materials of the
earth's crust, that its results are scarcely appreciable to the
ordinary observer. But it by no means follows that its effects
are not worthy of our consideration. This is particularly true
when we reflect that the results being discussed are not merely
those of days and weeks, but of years even when counted
by the tens of thousands and millions. Moreover, absolutely
pure water, as a constituent of our sphere, presumably does not
exist. We have to consider its action when contaminated with
sundry salts and acids which it has taken up in passing through
the atmosphere, and in filtering through the overlying layer of
organic matter and decomposition products which cover so large
a portion of the surface of the land. It is when thus contami-
nated that are manifested the wonderful solvent and other chem-
ical reactions which have been instrumental in promoting rock
destruction, and it is here, then, that will be considered the com-
plex chemical processes commonly grouped under the head of
oxidation, deoxidation, hydration, and solution'.

( 1 ) Oxidation. Oxidation is perceptibly manifested only in
rocks carrying iron either as sulphide, protoxide carbonate, or
silicate. The sulphides, in presence of water and when not
fully protected from atmospheric influences, readily succumb,
producing sulphates which, being soluble, are removed in solu-
tion, or hydrated oxides, sulphuretted hydrogen, and perhaps
free sulphur. Such an oxidation is attended by an increase in
bulk, so that if nothing escapes by solution, there may be brought
to bear a physical agency to aid in disintegration. Weathered
rocks, containing iron sulphides, may not infrequently be found
with cubical cavities quite empty or partially filled with the
brownish, yellow, or red product of its oxidation in a more or
less powdery condition. Pyrites, though a wide-spread constitu-
ent, is, nevertheless, a less conspicuous agent in promoting rock
decomposition than the protoxide carbonates and silicates. In

'See Walthers, Denudation in der Wuste, Vol. 16, No. 3, 1891, of the
Abhand. Matn.-phys. Cl. Konigl. Sachs. Gessell der Wiss. for details of
wind erosion in the Egyptian deserts.

166 THE PRINCIPLES INVOLVED IN ROCK-WEATHERING

these the iron also passes over to the hydrated sesquioxide state,
as is indicated by the general discoloration, the rock becoming
first streaked and stained, and finally uniformly ochreous. The
more common minerals thus attacked are the ferruginous car-
bonates of lime and magnesia, and silicates of the mica, amphi-
bole, and pyroxene groups. As the oxidation progresses, the min-
erals become gradually decomposed and fall away into unrecog-
nizable forms. The red and yellow colors of soils^are due to
the iron oxides contained by them. In some cases, the mineral
magnetite, a mixture of proto- and sesqui-oxides, undergoes
further oxidation and also loses its individuality.

(2) Deoxidation is a less common feature than oxidation.
"Water, carrying small quantities of organic acids, may take
away a portion of the combined oxygen of a sesquioxide, con-
verting it once more into the protoxide state. The local bleach-
ing of certain ferruginous sands and sandstones is due to this
action and to a partial removal of the ferriferous salt in solution.
Through a similar process of deoxidation, ferrous sulphates may
be converted into sulphides, a process which undoubtedly takes
place in marine muds protected from atmospheric action.

(3) Hydration 1 commonly accompanies oxidation, and, indeed,
is an almost constant accompaniment of rock decomposition, as
may be observed in comparing the total percentages of water in
fresh and decomposed minerals and rocks, as given in the
analyses.

This assumption, provided it be not accompanied by a loss of
constituents, either by solution or erosion, must be attended by
an increase in bulk, such as may be quite appreciable. The
Comte de la Hure, as quoted by Branner, 2 has expressed the
opinion that some of the hills of Brazil have actually increased
in height through this means. The present writer has calcu-
lated that the transition of a granitic rock into arable soil, pro-
vided the same took place without loss of material, must be
attended by an increase in bulk amounting to 88%. This ex-
pansion cannot, however, be attributed wholly to hydration.

Hydration as a factor in rock disintegration is, in the writer's

1 This word is here used in a more comprehensive sense than is customary,
and would include, in part at least, the hydrolysis of recent writers. See
Bell and Cameron, Bull. 30, Bureau of Soils, U. S. Dept. of Agriculture,
1905.

2 Op. cit., p. 284.

CHEMICAL ACTION OF WATEE 167

opinion, of more importance than is ordinarily supposed. Gran-
itic rocks in the District of Columbia have been shown 1 to have
become disintegrated for a depth of many feet with loss of but
comparatively small quantities of their chemical constituents
and with apparently but little change in their form for combina-
tion. Aside from its state of disintegration, the newly formed
soil differs from the massive rock, mainly in that a part of its
feldspathic and other silicate constituents have undergone a
certain amount of hydration Natural joint blocks of the rock
brought up from shafts were, on casual inspection, sound and
fresh. It was noted, however, that on exposure to the atmos-
phere such shortly fell away to the condition of sand. Closer
inspection revealed the fact that the blocks when brought to the
surface were in a hydrated condition, giving forth only a dull,
instead of clear, ringing sound, when struck with a hammer, and
showing a lustreless fracture, though otherwise unchanged.
That such had not previously fallen away to the condition of sand
was evidently due to the vice-like grasp of the surrounding rock
masses. These observations seem to have since received confir-
mation from Professor Derby, 2 who states that the sedimentary
rocks of Sao Paulo, Brazil, as seen in the deep railway cuttings,
"are almost invariably soft even when they show no signs of
decay, and go to pieces by a kind of slaking process when
broken up and exposed to the air, though they may have
required blasting in the original opening of the cuttings."
Professor W. O. Crosby* gives it as his opinion that the dis-
integration of the Pike's Peak (Colorado) granite is due mainly
to hydration, the mica particularly being affected.

Professor Alexander Johnstone showed 4 by experimentation
that normal muscovites, when submitted to the action of pure
and carbonated waters for the space of a year, underwent very
little change other than hydration, and a diminution in lustre,
hardness, and elasticity. They appeared, in fact, to be converted
merely into hydromuscovites, the hydration in pure water hav-
ing gone on nearly as rapidly as in that which was carbonated.
Biotite, when similarly treated, showed a slight discoloration
or bleaching on the edges, accompanied also by hydration, and,

l. GeoL Soc. of America, Vol. VI, p. 321.
* Decomposition of Rocks in Brazil, Jour, of GeoL, Vol. IV, 1896, p. 205.
3 Personal Memoranda to the Writer.
<Quar. Jour. Geol. Soc. of London, Vol. XLV, 1889.

168 THE PKINCIPLES INVOLVED IN KOCK-WEATHEEING

when carbonated water was used, a distinct loss of iron and
magnesia through solution. Lepidolite, voigtite, vermieulite, and
pyrosclerite were similarly acted upon, the iron and magnesia
being removed in the form of carbonates. The fact was noted
''that whenever anhydrous micas, or lower hydrated micas,
become hydrated, they always at the same time increase in bulk. ' '
This fact he regarded as accounting for the rapid weathering of
micaceous sandstones.

(4) Solution. The solvent action of water is perhaps the
most important of its immediate effects, though there are many
incidental chemical changes set in operation which, in the end,
are of equal or even greater significance. It is the solvent action
only that concerns us here.

As long ago as 1848 the Rogers brothers showed 1 that pure
water partially decomposed nearly all the ordinary silicate
minerals which form any appreciable part of our rocks. The
action of carbonated' water was recognizable in less than ten
minutes, but pure water required a much longer time before
its effect was sufficient for a qualitative determination. So pro-
nounced was the action of carbonated water that the presence of
lime, magnesia, and the alkalies could be recognized in a single
drop of the filtrate from the liquid in which the powdered min-
erals were digested. By digestion for forty-eight hours they
obtained from hornblende, actinolite, epidote, chlorite, serpen-
tine, feldspar, etc., a quantity of lime, magnesia, oxide of iron,
alumina, silica, and alkalies amounting to from 0.4% to 1% of
the whole mass. The lime, magnesia, and alkalies were ob-
tained in the form of carbonates ; the iron of the horn-
blende, epidote, etc., passing from the state of carbonate to that
of peroxide during the evaporation of the solutions. Forty
grains of finely pulverized hornblende, digested for forty-
eight hours in carbonated water at a temperature of 60, with
repeated agitation, yielded silica, 0.08%; oxide of iron,
0.095%; lime, 0.13%, and magnesia, 0.095%, with traces of
manganese. Commenting on these results, Bischof remarks 2
that "by repeating this treatment 112 times with fresh carbon-
ated water, a perfect solution might be effected in 224 days.
If now," he says, "40 grains of hornblende, unpowdered, in
which, according to the above assumption, the surface is only

1 Am. Jour, of Science, Vol. V, 1848.
. J Chemical and Physical Geology, Vol. I, p. 61.

W o

CHEMICAL ACTION OF WATER

169

one millionth of the powdered, were treated in the same way,
and the water renewed every two days, the time required for
perfect solution would be somewhat more than six million
years." In considering these figures and their practical bear-
ing, it must be remembered that while in nature the quantity
of water coming in contact with a crystal embedded in a rock
during a given time is much less than that assumed above, the
mineral is undergoing a gradual splitting up, becoming more
and more porous, so that the process is gradually accelerated.

To quote Bischof again, it is probably admissible to assume
that the time in which water produces similar effects of decom-
position or solution on minerals, is inversely as the magnitude of
the surface of contact. If, therefore, a mineral were so far subdi-
vided that the surface was increased ten million-fold, the quantity
then dissolved during a given time would be the same as that of
the undivided mineral during a period ten million times as long.

Richard Miiller has also shown 1 that carbonic acid waters
will act even during so brief a period as seven weeks upon the
silicate mineral with such energy as to permit a quantitative
determination of the dissolved materials. The accompanying
table from his paper shows (1st) the percentages of the various
constituents thus taken out by the carbonated water, and (2d)
the total percentages of the materials dissolved. That is to say,
the figures 0.1552 given for adular under Si0 2 , indicate that
0.1552% of the total 65.24% of the silica contained by the min-
eral have been removed, and so on. The last column gives the
total per cent of all the constituents extracted.

SOLUBILITY OF MINERALS IN CARBONIC ACID

MINERAL

8i0 2

Al a O,

K a O

Na,O

MgO

CaO

P.O.

FeO

Total

Adular .

%
0.1552

%
0.1368

%
trace

%
0328

Oligoclase .
Hornblende .

0.237
0.419

9.1713
trace

...

2.367

3.213

8528

trace
4.829

0.533
1.536

Magnetite .

trace

0942

0.307

Apatite . . .
Olivine .

0.873

trace

...

....

1.291

2.168
trace

1.822

8.733

2.018
2.111

Serpentine .

0.354

....

2.649

....

1.527

1.211

1 Untersuchen iiber die Einwirkung des kohlensaurehaltigen Wassers auf
einige Mineralien und Gesteine, Tschermaks Min. Mittheilungen, 1877, p. 25.

170 THE PEINCIPLES INVOLVED IN ROCK- WEATHERING

The summary of his investigations he gives as below:

(1) All the minerals tested were acted upon by the carbonated

water.

(2) In this process there were formed carbonates of lime, iron,

manganese, cobalt, nickel, potash, and soda.

(3) In the action of the carbonated waters upon the alkaline

silicates, like the feldspars, a small amount of silica went
always into solution, presumably in the form of hydrate.

(4) Even alumina was dissolved in appreciable quantities.

(5) Adular proved more resisting to the action of the acid than

did the oligoclase.

(6) The first stage of decomposition in the feldspars was a red-

dening process; the second, kaolinization

(7) Hornblende was more easily decomposed than feldspar.

(8) Increase of pressure on the solution was productive of more

energetic action than prolonging the time.
(9). Of all the minerals tested, the magnetic iron was least affected.

(10) Apatite was readily acted upon, as could be detected by its

appearance under the miscroscope.

(11) Olivine was the most readily attacked of all the silicates tested,

probably twice as easily decomposed as the serpentine.

(12) Magnesian silicates were attacked by the carbonated waters.

Hence serpentine cannot be considered a final product of
decomposition. 1

These and similar tests by more recent workers show with ap-
parent conclusivenesss that all the ordinary rock-forming miner-
als, silicates, oxides and carbonates are appreciably soluble
in the water of rainfalls and at ordinary temperatures.

Of all the materials forming any essential part of the earth's
crust the limestones are most affected. It is stated that pure
water will dissolve lime carbonate in the proportions of one part
in 10,800 when cold and one part in 8875 when boiling.

Since rock-weathering is, as already stated, a superficial
phenomenon, we have to do only with waters of ordinary tem-
peratures and under ordinary conditions of pressure, though
this expression must not be taken as necessarily meaning cold
waters, since, if we accept the statements of Caldcleugh, 2 rain
waters falling upon the heated rocks may have their tempera-
tures raised as high as 140 F. The enormously destructive
effect of carbonated waters on limestone is scarcely apparent

1 Serpentine, however, cannot be properly considered a decomposition
product. It is rather a product of alteration. See p. 107.

2 Trans. Geol. Soc. of London, 1829.

CHEMICAL ACTION OF WATER 171

on casual inspection, owing to the fact that the material is
carried away in solution, leaving only the insoluble impurities
behind. In such cases it is possible to estimate the amount of
corrosion through a comparison of the proportional amounts of
various constituents in this residue with those in the fresh rock
(see p. 217 et seq.), and the time limit of corrosion through
determining the percentage amounts of the constituents in the
water which annually drains from any given area. 1 By such
methods it has been estimated 2 that some 275 tons of calcium
carbonate are annually removed from each square mile of Cal-
ciferous limestone exposed in the Appalachian region alone;
while a well-known English authority 3 has calculated that with
an annual rainfall of 32 inches, percolating only to a depth of
18.3 inches, there are annually removed by solution from the
superficial portions of England and Wales an average of all
constituents amounting to 143.5 tons per square mile of area.
He further calculates that the average amount of carbonate of
lime annually removed from each square mile of the entire
globe amounts to 50 tons. 4 It is to this corrosive action of

x The following calculations by Sir John Murray show the amount and
kind of material in solution in one cubic mile of average river water:

Constituents Tons in one cubic mile

Calcium Carbonate (CaCO 3 ) 327,710

Magnesium Carbonate (MgCO 3 ) 112,870

Calcium Phosphate (Ca 3 P 2 O 8 ) 2,913

Calcium Sulphate (CaSO 4 ) 34,361

Sodium Sulphate (NaSO 4 ) 31,805

Potassium Sulphate (K,SO 4 ) 20,358

Sodium Nitrate (NaNO 3 ) 26,800

Sodium Chloride (NaCl) 16,657

Lithium Chloride (LiCl) 2,462

Ammonium Chloride (N- H 4 C1) 1,030

Silica (Si0 2 ) 74,577

Ferric Oxide (Fe 2 O 3 ) 13,006

Alumina (A1 2 O 8 ) 14,315

Manganese Oxide (Mn 2 O 3 ) 5,703

Organic Matter 79,020

2 A. L. Ewing, Am. Jour, of Science, 1885, p. 29.

8 T. Mellard Eeade, Chemical Denudation in Eelation to Geological Time.

4 The total dissolved constituents thus removed are divided up as follows :
Carbonates of lime, 50 tons; sulphate of lime, 20 tons; silica, 7 tons; car-
bonate of magnesia, 4 tons; peroxide of iron, 1 ton; chloride of sodium,
8 tons; alkaline carbonates and sulphates, 6 tons.

172 THE PEINCIPLES INVOLVED IN EOCK- WEATHERING

meteoric waters that still another authority 1 would attribute
the slight thickness and nodular condition of many beds of
Palaeozoic limestone. He argues that originally thick-bedded
limestones have, during the ages subsequent to their formation
and uplifting, become so impoverished through the dissolving
out and carrying away in solution of the lime carbonate, as to
have been quite obliterated, or reduced to mere nodular bands,
and given rise to important palaaontological breaks in the geo-
logical record. Other than organic acids may locally exert a
potent influence. Thus Eobert Bell has described the dolomitic
limestones underlying the waters along Grand Manitou Island,
the Indian peninsula, and adjacent portions of Lake Huron an,d
the Georgian Bay, as pitted and honeycombed in a very pecu-
liar and striking manner. This corrosion, it is believed, is
produced through the solvent action of sulphuric acid in the
water, the acid itself arising from the decomposition of the sul-
phides of iron, pyrites and pyrrhotite, which exist in great
quantities in the Huronian rocks to the northward. 2

1 F. Eutley, The Dwindling and Disappearance of Limestones, Quar. Jour.
Geol. Soc. of London, August, 1893.

2 Bull. Geol. Soc. of America, Vol. VI, pp. 47-304.

Messrs. C. W.^ Hayes and M. E. Campbell, of the United States Geological
Survey, have reported some remarkable examples of corroded quartz pebbles
which should be mentioned here, although a satisfactory explanation for
the phenomenon has not yet been given.

Dr. Hayes, in a personal memorandum to the writer, describes the occur-
rence as follows:

"At three rather widely separated points in the South, conglomerates
have been observed in which the projecting portions of the pebbles have
been etched or partly dissolved.

' ' The first, observed by Mr. Campbell, is at Nuttall, West Virginia. The
conglomerate in question, which belongs to the coal measures, is composed
of rather coarse quartz sand with slightly yellowish cement, in which are
embedded well-worn pebbles of white vein quartz. The latter vary in size
up to three-quarters of an inch in diameter, and are somewhat irregularly
distributed. Ordinarily the pebbles, wholly unaltered, weather out by the
chemical or mechanical disintegration of the sandy matrix. In the case
observed, however, where the conglomerate received the drip from an over-
hanging cliff, the projecting portions of the pebbles are deeply pitted evi-
dently by solution. Mechanical wear is precluded by the form of the re-
sulting surface, which is not smooth like the portions of the pebble still pro-
tected by the matrix, but is rough and irregular. The outer portion of the
pebbles is evidently less easily affected by the solvent than the interior,
and forms a sharp rim about the regular cavities hollowed out within. In
some cases a third of the pebble has thus been removed. The surface of

CHEMICAL ACTION OF WATER 173

The relative solvent power of salt and fresh water has often
been discussed, and in some cases actual tests have been made.
Thoulet 1 obtained the results given below, the tests extending
over a period of but 24 hours. Though not conclusive, they seem
to show that, so far as the particular materials tested are con-
cerned, fresh water is by far the more energetic. The figures
give the loss in grams from a cubic centimeter.

Shell Coral Geobigerina

Pumice limestone rock ooze

Sea water 0.000105 0.000039 0.000201 0.000137

Fresh water .... 0.000832 0.001843 0.003014 0.003091

the sandstone matrix in which the pebbles are embedded is also pitted,
possibly by the same process of solution as that which has affected the
pebbles, but such a surface might also be produced by mechanical means in
case the cement were less indurated in some places than in others.

"The second case is on Clifty Creek, White County, Tennessee. The con-
glomerate, also a member of the coal measures, forms the bottom of a small
canon, and is covered by the creek at high water, but uncovered throughout
the greater part of the year. The matrix is a coarse white sandstone which
weathers yellow by the oxidation of the slightly ferruginous cement. Em-
bedded in this are rather abundant pebbles, varying in size up to two inches
in diameter, and composed chiefly of quartz, with a few of chert and pos-
sibly of quartzite. The projecting portions of these pebbles have been in
part removed, though they still project somewhat above the enclosing matrix.
As in case of the Nuttall conglomerate, the exterior portions of the pebbles
are less easily affected than the interiors, and when the pebble has been
a third or half removed the outer shell forms a rim within which is a de-
pression with a slight elevation in the centre. The chert pebbles show less
evidence of corrosion by a solvent than those composed of quartz. Their
upper surfaces are somewhat worn down and even slightly hollowed, but
this might easily have been produced by mechanical means, which is not
the case with quartz.

' ' The third case is a block of conglomerate from Starrs Mountain, Ten-
nessee, collected by Mr. Bailey Willis. This is of Lower Cambrian age.
The matrix is a coarse feldspathic sandstone containing layers of well-
rounded pebbles, mostly quartz, with a few probably of some feldspar. The
former are between one-half and one inch in diameter and the latter some-
what larger. The projecting portions of the quartz pebbles on one side of
the block are almost entirely removed, and as in the other cases evidently
by solution. A slight rim projects above the matrix in which the pebbles
are embedded; within this is a depression, while a slight elevation occupies
the centre.

' ' The projecting portions of the feldspathic pebbles also are partly re-
moved, but this may be due to corrasion instead of corrosion, that is, to the
action of mechanical rather than chemical agents. The pebbles on the
lower side of the block have their original water-worn surfaces without any
trace of etching. ' '

1 Comptes Eendus Paris Academic, Vol. 110, 1890, p. 652.

174 THE PEINCIPLES INVOLVED IN BOCK-WEATHERING

Daubree's experiments, noted on page 176, showed also
decomposition to be retarded by the presence of sodium chloride
in solution.

These results, however, do not at all agree with those obtained
in a carefully conducted series of experiments by Prof. Joly,
who showed 1 that under the same conditions sea-water dissolved
from hornblende, orthoclase, obsidian and basalt from two to
fourteen times as much material as did the fresh water.

It has in times past been very generally assumed that certain
complex, unstable, and little understood organic compounds,
known under such names as humic, ulmic, crenic, and apocrenic
acid were present in soils rich in organic matter, and that, fur-
ther, such might be of considerable geological significance. 2 Ee-
cent studies by Cameron and Bell, 3 however, throw a doubt on
the very existence of these acids. Even do they exist, their
solvent action is shown to be quite insignificant, and it is re-
garded as probable that results heretofore attributed to these
agencies are in reality due to carbonic acid. 1

iProc. Boyal Irish Academy, Vol. 24, 1902.

2 See A. A. Julien, The Geological Action of Humus Acids, Proc. Am.
Assoc. for the Adv. of Science, 1879, p. 324.

3 Bull. 30, Bureau of Soils, U. S. Dept. of Agr., 1905.

<Berthelot and Andre (Comptes Eendus Academie de Paris, 114, 1892,
pp. 41-32) have shown that the brown substance of humus and analogous
compounds undergo direct oxidation under the influence of the air and sun-
light, forming carbonic acid. These reactions take place without the inter-
vention of microbes, and are accompanied by a change in color of the original
humus. The oxidation is rendered more active through the division and
mellowing of the humus by cultivation. Through chemical union of the
carbonic acid with certain bases, as lime, soda, and potash, there are formed
soluble carbonates which may be leached out by meteoric waters.

The writer was shown not long since, by Professor Charles E. Munro, a
very practical illustration of the remarkable corrosive power of organic
acids. A highly ornate French clock, with case of black marble, was packed
for storage in excelsior which was a trifle damp. The clock remained in
storage from the last of May until about the first of October of the same
year. When the packing material was removed, the marble was found to
be so corroded as to need rehoning and polishing. The roughness could be
easily felt by passing the finger over the surface, and long lustreless lines
indicating the contact of excelsior fibres traversed the surface in every
direction.

PLATE 15

FIG. 1. Diorite boulder split along joint planes by frost.
FIG. 2. Exfoliated granite boulder. U. S. G. S.

MECHANICAL ACTION OF WATER AND ICE 175

3. MECHANICAL ACTION OF WATER AND OF ICE

Aside from its solvent capacity, water acts as a powerful ero-
sive agent, as well as an agent for the transportation of the
eroded materials. It is only its erosive power that need con-
cern us here, though, as will be seen, this is to a considerable
extent dependent upon its power of transportation. Every
raindrop beating down upon a surface already sorely tried by
heat and frost serves to detach the partially loosened granules,
and, catching them up in the temporary rivulets, carries them
to the more permanent rills, to be spread out over the valley
bottoms, or perhaps, if the slopes be steep and the current ac-
cordingly strong, to the rivers and thence to the sea. The
amount of detrital matter thus mechanically removed from
the hills and spread out over valley and sea-bottoms quite ex-
ceeds our comprehension, but it is estimated that at the rate
the Mississippi River is now doing its work, the entire Ameri-
can continent might be reduced to sea-level within a period of
four and one-half million years. The Appalachian Mountain
system, the uplifting of which began in early Cambrian times
and terminated at the close of the Carboniferous, has already
through this cause lost more material than the entire mass of
that which now remains. But the rivers, like the winds and
glaciers, in virtue of this load they bear, become themselves
converted into agents of erosion, filing away upon their rocky
beds, undermining their banks, and continually wearing away
the land by their ceaseless activity. The pot-holes in the bed
of a stream, formed by the swirl of sand and gravel in an
eddy, furnish on a small scale striking illustrations of this
cutting power, while the rocky canons of the Colorado of the
West, where thousands of feet of horizontal strata have been cut
through as with a file, show the same thing on a scale so gigantic
as to be at first scarce comprehensible. 1 An item of no in-
significant importance to be considered here is the possibility,
indeed probability, of an incidental chemical decomposition
taking place during this abrasive action. Daubree showed 2

1 Captain C. E. Button has estimated (Tertiary History of the Grand
Canon of the Colorado) that from over an area of 13,000 to 15,000 square
miles drained by the Colorado Kiver, an average thickness of 10,000 feet of
strata have been removed.

2 It will be remembered that this authority placed rock fragments in stone
and iron cylinders containing water and made to revolve horizontally at a

176 THE PRINCIPLES INVOLVED IN ROCK-WEATHEEING

that when feldspathic fragments were submitted to artificial
trituration in a revolving cylinder containing water, a decompo-
sition was effected whereby the alkalies were liberated in very
appreciable amounts. He found further that the principal
product of mutual attrition of feldspar fragments in water was
not sand, but an impalpable mud (limon). This mud was of
such tenuity as to remain for many days in suspension, and
on desiccation became so hard as to be broken only with
the aid of a hammer, resembling in many respects the argil lites
of the coal measures, but differing in that it carried a high
percentage of alkalies. Granitic rocks thus treated yielded
angular fragments of quartz and very minute shreds of mica,
while the feldspars ultimately quite disappeared in the form
of the impalpable mud above mentioned. It was noted that
after the quartzose particles had reached a certain degree of
fineness further diminution in the size ceased, owing to the
buoyant action of the water, which in the form of a thin film
between adjacent particles acted as a cushion and prevented
actual contact tc the extent necessary for mutual abrasion. It
is to similar action on the part of sea-water that Shaler 1 would
attribute the lasting qualities of the sand grains upon sea
beaches. Indeed the conditions of Daubree's experiments as
a whole were not so different from those existing in nature that
one need hesitate, as it seems to the writer, to conclude similar
action, both chemical and physical, may be going on wherever
abrasion takes place in the presence of continual moisture, as in
the bed of a river or glacier.

The hammering action of waves upon the sea-coast exerts a
powerful erosive action, particularly upon particles of rock of

measured rate of speed, so that the actual distance travelled by any of the
particles during a given time could be readily calculated. The product of
this disintegration, even when carried to the condition of fine silt, was always
sharply angular. His experiments further showed that when feldspathic
fragments were thus treated, there was always a certain amount of decom-
position, whereby salts of potash were liberated; in one instance, when 3
kilogrammes of feldspar were revolved for 192 hours in iron cylinders con-
taining 5 litres of water, 2.72 kilogrammes of finely comminuted mud were
obtained, and in solution in the water, 12.6 grammes of potash, or 2.52
grammes per litre. The presence of carbonic acid in the water increased
the amount of potash. When the feldspar was triturated dry and then
treated with water, no such solvent action could be detected. Geologic Ex-
perimentale, p. 268.

1 Bull. Geol. Soc. of America, Vol. V, p. 208.

MECHANICAL ACTION OF WATER AND ICE 177

such size as to be lifted or moved by wave action, but too heavy
to be protected from attrition by the thin film of water above
alluded to. Shaler's observations 1 at Cape Ann were to the
effect that ordinary granitic paving blocks (weighing perhaps
twenty pounds) were, when exposed to surf action, worn in the
course of a year into spheroidal forms such as to indicate an
average loss of more than an inch from their peripheries. Even
the crystallization of the salt thrown up by wave action and ab-
sorbed into the pores of rocks serves in its way the purposes of
disintegration. 2

The Action of Freezing Water and of Ice. The action of
dry heat and cold in disintegrating rocks has already been
described. The effects of such temperature changes upon
stone of ordinary dryness are, however, slight in comparison
with the destructive agencies of freezing temperatures upon
stones saturated with moisture. The expansive force of water
passing from the liquid to the solid state has been graphically
described as equal to the weight of a column of ice a mile high
(about 150 tons to the square foot). Otherwise expressed, 100
volumes of water expand, on freezing, to form 109 volumes of
ice. Provided, then, sufficient water be contained within the
pores of a stone, it is easy to understand that the results of
freezing must be disastrous. That stones as they lie in the
ground do contain moisture, often in no inconsiderable amounts,
is a well-known and well-recognized fact by all those engaged
in quarrying operations, and indeed no mineral substance is
absolutely impervious to it. The amount contained, naturally
varies with the nature of the mineral constituents and their
state of aggregation. According to various authorities, granite
may contain some 0.37% by weight; chalk, 20%; ordinary
compact limestone, 0.5% to 5% ; marble, about 0.30% ; and
sandstones, amounts varying up to 10% or 12%, while clay
may contain nearly one-fourth its weight. This water is largely
interstitial the quarry water, as it is sometimes called. In
addition to this, the quartz, particularly of granitic rocks, almost
universally contains minute cavities partially filled with water,

'Bull. Geol. Soc. of America, Vol. V, p. 208.

2 According to Dana (Wilkes' Exploring Expedition, Geology, p. 529), the
sandstones along the coast of Sydney, Australia, are subjected to a mechani-
cal disintegration through the crystallization of salt which is absorbed from
the saline spray of the ocean waves.

178 THE PRINCIPLES INVOLVED IN ROCK- WEATHER ING

which, in extreme cases, are so abundant as to make up, accord-
ing to Sorby, at least 5% of the whole volume of the mineral.

That the passage of this included moisture from the liquid
to the solid state, must be attended with results disastrous to
the stone is self-evident, though the rate of disintegration may
be so slow under favorable circumstances as to be scarce notice-
able. Freezing of the absorbed water is one of the most fruit-
ful sources of disintegration in stones confined in the walls of
a building, and even in the quarry bed it is by no means uncom-
mon to have stone so injured as to render it worthless. How-
ever slight may be the effects of a single freezing, constant
repetition of the process cannot fail to open up new rifts, and
still further widen those already in existence, allowing further
penetration of water to freeze in its turn and to exert a chemical
action as well. So year in and year out, through winter's cold
and summer's heat, the work goes on until the massive rock
becomes loose sand to be caught up by winds or temporary
rivulets and spread broadcast over the land. In some instances,
it may be, the rock is of sufficiently uniform texture to be af-
fected in all its mass alike. More commonly, however, it is
traversed by veins, joints, or other lines of weakness along
which the rifting power is first made manifest, as in the illustra-
tion. (PI. 19.) Naturally disintegration of this kind is con-
fined to frigid and temperate latitudes. As bearing upon the
extreme rapidity with which such disintegration may take place,
the following is quoted from a letter of Dr. L. Stejneger, of the
United States National Museum, who passed several months
among the islands of Bering Sea.

"In September, 1882, I visited Tolstoi Mys, a precipitous
cliff near the southeastern extremity of Bering Island. At the
foot of it I found large masses of rock and stone which had
evidently fallen down during the year. Most of them were
considerably more than six feet in diameter, and showed no
trace of disintegration. The following spring, April, 1883,
when I revisited the place, I found that the rocks had split up
into innumerable fragments, cube-shaped, sharp-edged, and of
a very uniform size, about two inches. They had not yet
fallen to pieces, the rocks still retaining their original shape.
I may remark, however, that the weather was still freezing when
I was there. The winter was not one of great severity, and several
thawing spells broke its continuity. These cubic fragments did

MECHANICAL ACTION OF WATEK AND ICE 179

not seem to split up any further, for everywhere on the islands
where the rock consisted of the coarse sandstone, as in this place,
the talus consisted of these sharp-edged stones."

Ice acts as a disintegrating agent in still other ways than
that mentioned. The phenomenon of the glacier is now so
well known that we need dwell upon it but briefly here. Long-
continued precipitation of snow upon regions of such elevation,
or in such latitudes as to preclude anything like an equally
rapid melting, gives rise to deep fields of snow, compacted in
the lower portions into the condition of ice. Advancing, it may
be, but an inch or several feet a day, now scarce moving at all,
or even retreating temporarily through a diminution in the
amount of their supplies, or an increase in the sun's heat, these
carry with them large quantities of fragmental rock material
fallen upon them from above, or picked up from the surfaces
over which they flow. Those fragments which remain upon the
upper surface, or frozen into the upper portions, are but trans-
ported to the lower levels where, the temperature being suffi-
cient, the ice is melted and the load deposited in the form of a
moraine.

Beneath, and frozen into the lower portion of the ice sheet,
there is, however, a variable amount of rock material, which, as
the glacier moves along, is crowded with all the weight of the
overlying mass, and all the resistless energy of the ice behind,
over the surface of the underlying rock. In virtue of this
material, this sand, gravel, and boulder aggregate, the glaciers
become converted into what we may compare to extremely
coarse files, to tear away the rocks over which they pass, and
grind and crush them into detritus of varying degrees of
fineness. The small streams which originate from the melting
of the glaciers become therefore charged to the point of tur-
bidity with the fine silt-like detritus ground from the ledges
and in part from the boulders themselves. Figure 3 of plate 20
shows a slab of limestone still bearing upon its surface the evi-
dences of the severity of the onslaught. A consideration of the
amount of detritus thus brought down either merely as transported
or as abraded material belongs properly to the chapter on trans-
portation, but a few illustrations are not without interest here.
The Aar in Switzerland is stated by Geikie to discharge every day
in August some 440,000,000 gallons of water, carrying some
280 tons of sand. A portion of this is in a state of such

180 THE PEINCIPLES INVOLVED IN ROCK-WEATHERING

minute subdivision as to remain a long time in suspension,
and give the water a milky appearance for several miles.
I. C. Russell has described 1 the Tuolurnne Eiver, issuing from
the foot of the Lyell Glacier in the Sierras of California, as
turbid with silt which has been ground by the moving ice.

At the foot of the Dana Glacier there is a small lakelet
whose waters are of a peculiar greenish yellow color from
the silt held in suspension, and which, when submitted to
microscopic examination, is found to be made up of fresh
angular fragments of various silicate minerals of all sizes from
0.35 mm. in diameter down to impalpable silt.

4. ACTION OF PLANTS AND ANIMALS

Both plants and animals aid to some extent in the work of
rock disintegration. Plants are also important factors in pro-
moting sedimentation, while burrowing insects and animals may
exert an important influence upon the texture of soils and in
bringing about a more general admixture by transferring to the
surface that which is below.

The lower forms of plant life, the lichens and mosses,
growing upon the hard, bare face of rocky ledges send their
minute rootlets into every crack and crevice, seeking not merely
foot-hold, but food as well.

Slight as is the action, it aids in disintegration. The plants
die, and others grow upon their ruins. There accumulates thus,
it may be with extreme slowness, a thin film of humus, which
serves not merely to retain the moisture of rains and thus bring
the rock under the influence of chemical action, but supplies at
the same time small quantities of the organic acids to which
reference has already been made. These act both as sol-
vents and deoxidizing agents. As time goes on, sufficient
soil gathers for other, larger and higher types of life, which exert
still more potent influences. It may be the rock is in a jointed
condition. Into these joints each herb, shrub, or sapling pushes
down its roots, which, in simple virtue of their gain in bulk, day
by day, serve to enlarge the rifts and furnish thereby more ready
access for water, and the wash of rains, to still further augment
disintegration. The depth to which such roots may penetrate has
often been noted, varying, as is to be expected, with the nature

1 5th Ann. Rep. U. S. Geol. Survey, 1883-84.

ACTION OF PLANTS AND ANIMALS 181

of the soil. Aughey has found roots of the buffalo berry
(Shepherdia argophylla) penetrating the loess soils of Nebraska
to the depth of 50 feet. In the limestone caverns of the Southern
States, the writer has often been impressed by the number of long
thread-like rootlets, so fine as to be almost imperceptible, which
have found their way through rifts in the rocky roof.

H. Carrington Bolton has shown that very many minerals
are decomposed by the action of cold citric acid for a more or
less prolonged period, the zeolites and other hydrous silicates
being especially susceptible. Such tests have great significance
when we consider that the roots of growing plants secrete an
acid sap, which, by actual experiment, has been found capable
of etching marble. The exact nature of this acid is not accurately
known, but it is considered probable that in the rootlets of each
species of plant there exists a considerable variety of organic
acids. 1

But the effects of plant growth are not necessarily always
destructive; such may be conservative or even protective. In
glaciated regions, it is often the case that the striated and pol-
ished surfaces of the rocks have been preserved only where pro-
tected from the disintegrating action of the sun and atmosphere
by a thin layer of turf or moss. As a general rule, however,
the manifest action of plant growth is to accelerate chemical
decomposition, through keeping the surfaces continually moist,
and to retard erosion.

Action of Bacteria. The researches of A. Miintz, 2 Wido-
gradsky, Schlosing, and others tend to show that bacteria may
exercise an important influence in promoting rock disintegra-
tion and decomposition. Their influence in promoting nitri-
fication has been already alluded to. It would appear that
while these organisms secrete and utilize for their sustenance
the carbon from the carbonic acid of the atmosphere, as do
plants of a higher order, they may also assimilate carbonate
of ammonium, forming from it organic matter and setting free
nitric acid. Being of* microscopic proportions, the organisms

1 See Application of Organic Acids to the Examination of Minerals, H.
Carrington Bolton, Proc. Am. Assoc. for the Advancement of Science, XXXI,
1883, and Available Mineral Plant Food in Soils, B. Dyer, Jour. Chem.
Society, March, 1894. Recent work seems to show that the corrosive effect
of root action as noted above may have been due wholly to carbonic acid.
See Bull. 30, Bureau of Soils, U. S. Dept. of Agriculture.

Comptes Eendus de 1 'Academic des Sciences, CX, 1890, p. 1370.

182 THE PRINCIPLES INVOLVED IN ROCK-WEATHEBING

penetrate into every little cleft or crevice produced by atmos-
pheric agencies, and throughout long periods of time produce
results of no inconsiderable geological significance. The depth
below the surface at which such may thrive is presumably but
slight, and their period of activity limited to the summer months.
They have been found on rocks of widely different character
granites, gneisses, schists, limestones, sandstones, and volcanic
rocks an( j on high mountain peaks as well as on lower levels.
The Pic Pourri, or Rotten Peak of the Bernese Alps, is cited as
composed of friable and superficially decomposed calcareous
schists, throughout the whole mass of which are found the nitri-
fying bacteria, which are believed to have been instrumental in
promoting its characteristic decomposition. The organism acts
even upon the most minute fragments, reducing them continually
to smaller and smaller sizes. Each fragment loosened from the
parent mass is found coated with a film of organic matter thus
produced, and the accumulation begun by these apparently in-
significant forces is added to by residues of plants of a higher
order, which come in as soon as food and foothold are provided. 1
Mr. J. E. Mills, 2 and after him J. C. Branner, 3 lay stress on
the decomposing effect of vegetable matter carried into the
ground by ants in certain parts of Brazil, Mills going so far as
to describe the ants as continually pouring carbonic acid into
the ground. Be this as it may, the excretions of the ants them-
selves are undoubtedly of such a nature as to further the proc-
esses of decomposition. Certain species of ants, locally known
as saubas, or sauvas, live, according to Prof. Branner, in enor-
mous colonies, burrowing in the earth, where they excavate cham-
bers with galleries that radiate and anastomose in every direction,
and into which they carry great quantities of leaves. Certain
species of termites, the white ants of Brazil are also active pro-

1 It is, perhaps, as yet, too early to say to what extent the presence of
bacteria may be incidental to decomposition, rather than causative. Branner
in a recent summary of this subject (Am. Jour. Sei., Vol. Ill, 1897, p. 442),
says: "In other words nitrifying bacteria not only do not penetrate the
rocks themselves to any considerable depths, but they do not even penetrate
the soil to a depth of more than three or four feet. In the face of this
fact, and the other fact that our granites are often decomposed to depths
of more than 100 feet, it seems quite improbable, if not impossible, that
bacteria are responsible for this deep decay, or for any considerable part
of it."

2 American Geologist, June, 1889, p. 357.

3 Bull. Am. Geol. Soc. of America, Vol. VII.

ACTION OF PLANTS AND ANIMALS 183

moters in bringing about changes in the structure of the soil,
and incidentally accelerating decomposition. The organic matter
carried by these creatures into the ground, there to decompose,
furnishes organic acids to promote further decay in the material
close at hand, and by its downward percolation to attack the
still firm rocks at greater depths. Indeed, these numerous chan-
nels, through affording easy access of air and surface waters with
all their absorbed gases or alkaline salts, may serve indirectly a
geological purpose scarcely inferior to that of the joints in
massive rocks. (See further under soil modified by plant and
animal life.)

The mechanical agency which has already been referred
to as instrumental in bringing about a certain amount of de-
composition in silicate minerals, is greatly augmented when such
trituration takes place in connection with organic matter. J. Y.
Buchanan has shown, 1 that the mud of sea-bottoms is being
continually passed and repassed through the alimentary canals
of marine animals, and that in so doing the mineral matter not
merely undergoes a slight amount of comminution and consequent
decomposition, but a chemical reduction takes place whereby
existing sulphates are converted into sulphides. Such sulphides
and the metallic constituents of the silicates and other compounds
particularly those of iron and manganese, would on exposure be-
come converted into oxides. It is through these agencies that he
would account for the presence of sulphur in marine muds, and
the variations in color, from shades of red or brown to blue and
gray, in the former the iron occurring as oxides, while in the lat-
ter it exists as a sulphide. Of course either form may be more or
less permanent according as the mud may be devoid of animal
life, or protected from oxidizing influences. These reactions,
being subaqueous, are somewhat beyond the scope of the present
work, but are nevertheless not without interest in this connection.

It is further to be noted that the solvent and general chemical
activity of water is often greatly augmented by the salts and
acids it acquires through the decomposition of various minerals
with which it comes in contact. Through the decomposition of
iron pyrites there may be formed free sulphuric acid, or through
the decomposition of a feldspar, carbonates of the alkalies, any
of which, when in solution, are more energetic factors in pro-

1 On the Occurrence of Sulphur in Marine Muds, Proc. Royal Soc. of
Edinburgh, 1890-91.

184 THE PEINCIPLES INVOLVED IN EOCK-WEATHEEING

moting decomposition than water alone. Hence under certain
conditions the process of decomposition once set in operation
augments itself, and goes on with increasing vigor until such a
depth is reached that the percolating solutions become neutralized
and further action, aside from hydration, practically ceases.

PLATE 16

Weathered granite, District of Columbia.

THE WEATHERING OP ROCKS (Continued)

II. CONSIDERATION OF SPECIAL CASES

Let us now enter into a consideration of the composition of
a few prominent rock types, and note the changes they have
undergone in this process of weathering, assuming, as we must
for the time being, that they have been all subjected to essen-
tially the same conditions. Inasmuch as there are divers types
of rocks, differing not merely in chemical and mineral composi-
tion, but in structure as well, it is an easy assumption that the
results of prolonged weathering may be widely divergent. Yet,
as will become apparent, the ultimate products from all but the
purely quartzose rocks, present striking similarities.

Weathering of Granite. In the tables following are given
the results of chemical and mechanical analyses of rocks of
various kinds and in varying stages of degeneration. We will
begin with a consideration of the granitic rocks of the District
of Columbia. 1 The climate of the region, it should be stated, is
somewhat capricious, the Weather Bureau records showing ex-
treme ranges of 15 to + 104 Fahr., while an annual range
from 10 to 95 is common. The average annual temperature is
54.7 Fahr., and the average precipitation 43.96 inches.

The rock in its fresh condition is a strongly foliated gray
micaceous granite showing to the unaided eye a finely gran-
ular aggregate of quartz and feldspars arranged in imperfect
lenticular masses from 2 to 5 mm. in diameter, about and
through which are distributed abundant folia of black mica.
In the thin section the structure is seen to be cataclastic.
Quartz and black mica are the most prominent constituents,
though there are abundant feldspars of both potash and soda-
lime varieties, which, owing to their limpidity, can by the
unaided eye scarcely be distinguished from the quartz. The
potash feldspar has in part a microcline structure. Aside from
these minerals, a primary epidote, in small granules and at times

1 Disintegration of the Granitic Eocks of the District of Columbia, Bull.
Geol. Soc. of America, Vol. VI, 1895, pp. 321, 332.

185

186

ROCK DISINTEGRATION AND DECOMPOSITION

quite perfectly outlined crystals, is a strikingly abundant con-
stituent Small apatites, a few flakes of white mica (sericite),
and widely scattering black tourmalines and iron ores complete
the list of recognizable minerals.

The outcrops from which the samples for the analyses to
which attention is first called were selected are shown in plate
16. At the very bottom, the rock is hard, fresh, and com-
pact, without trace of decomposition products other than as
indicated by minute infiltrations of calcite from above. Just
above the level of the small creek which flows at the foot of
the bluff at the point indicated by the first series of right-and-
left joints near the centre of the view, the character of the rock
changes quite suddenly, becoming brown and friable, though
still retaining its form and easily recognizable granitic appear-
ance. A few feet above a third zone begins, in which the rock
is converted into sand and gravel, which becomes more and
more soil -like to the top of the bank, where it becomes admixed
with organic matter from the growing plants. The amount
of organic matter is quite small, however, and in making the
analyses care was taken to remove such as was recognizable in
the form of rootlets, leaves, and twigs.

ANALYSES OF FRESH AND DECOMPOSED GRANITE, DISTRICT OP COLUMBIA

CONSTITUENTS

III

Ignition

1.22 L

3.27 %

4.70%

Silica (Si0 2 )

69 33

66 82

65 69

Titanium (Ti0 2 )

not det

Alumina (A1 2 3 )
Iron protoxide (FeO)

14.33
3.60 1

15.62
1 69

15.23

Iron sesquioxide (Fe 2 Os)

1 88

4 39

Lime (CaO) ....

3 21

3 13

2 63

Magnesia (MgO) ...

2 44

2 76

2 64

Soda (Na 2 O)

2 70

2 58

2.12

Potash (K 2 O) .

2 67

2 04

2 00

Phosphoric acid (P 2 O 6 ) ....

0.10

not det.

0.06

09.60 %

99.79%

99.77 %

Bulk analyses of these three types, (I) fresh gray granite,
(II) brown but still moderately firm and intact rock, and (III)
'4.00% when calculated as Fe 2 O 8 .

WEATHERING OF GRANITE 187

the residual sand, yielded the results given in the columns cor-
respondingly numbered on the preceding page.

In glancing over. these figures it is at once apparent that
there is a surprisingly small difference in ultimate composition
between the sound rock and the residual sand, the more marked
differences being a slightly smaller amount of silica, more alu-
mina, and slightly diminished amounts of lime, magnesia, pot-
ash, and soda, with a considerable increase in the amount of
water. The ferrous salts have moreover been converted into
ferric forms. It does not necessarily follow, however, that no
more actual gain or loss of material or change in manner of
combination than is here indicated may not have taken place,
and at the very outset it may be well to enter into a discussion
of the manner in which the results of such analyses are to be
considered.

We must first of all remember that any indicated loss or
gain of a constituent may be only apparent, and that the true
relative proportions can be learned only by calculating results
of analyses of both fresh and decomposed materials on a com-
mon basis Thus the first glance at analysis III, as given,
might lead one to surmise that the decomposed rock had actually
lost 'only some 3.64% of silica. This, however, is not strictly
the case, since this analysis shows 4.7% volatile constituents
against 1.22% in analysis I of the fresh material. Could we
assume that this difference of 3.48% was due wholly to a
uniform absorption of moisture, as by a clay, the problem would
resolve itself into simply recalculating all analyses upon a
water-free basis.

The results obtained thus are not quite satisfactory, however,
and it is thought a more correct view of the changes taking
place may be obtained by assuming for one of the constituents
a fairly constant value and using this as a basis for comparison.

Of all the essential constituents occurring in appreciable
quantities in siliceous crystalline rocks the alumina and the iron
oxides are the most refractory and the least liable to be removed
by a leaching process, although they may undergo manifold
changes in mode of combination. Although not absolutely
correct, therefore, we will for our present purposes assume the
one or, the other of these (in this case the iron as Fe 2 3 ) as a
constant factor, and in order to show the proportional or actual
amount of loss of any constituent will recalculate the analyses

188 KOCK DISINTEGRATION AND DECOMPOSITION

upon this basis, a proceeding for which, so far as alumina is
concerned, we have already good authority. 1 This method will
be adopted, however, only with the siliceous crystalline rocks,
in which, for reasons noted later, the process of decomposition,
we have reason to suppose, is more complex than in calcareous
and magnesian rocks poor or lacking in the alkalies. The
entire discussion is one beset with great difficulties, since we
lack definite knowledge as to the exact processes which have
been going on and need constantly to guard against assump-
tions too hastily drawn or based upon insufficient data. Indeed,
any assumption based upon the results of chemical analyses
alone is likely to lead to grave error. We may be certain, how-
ever, that the estimates as thus obtained are invariably too low,
since it is not possible to conceive of decay in which even the most
refractory constituent is not carried away in appreciable quan-
tities. "Whether the iron or the alumina remains most nearly
constant must depend upon local conditions.

If, then, in this particular case, the iron in the form of Fe 2 O 3
is considered a constant factor, by proper calculation we obtain
the results given in column (IV) on p. 189, which represent the
proportional gain and loss of the various constituents of the rock
in passing from the condition indicated in column (I) on the pre-
ceding page, to that indicated in column (III). Such a com-
parison is instructive as showing not merely the relative loss and
gain, but also the total loss of material, in this case 13.79%, ac-
companied by a gain of 2.16%, in volatile matter.

Such results are still far from satisfactory, and it is believed
the tables will be more useful and instructive can we show the
percentage loss and gain of each constituent as compared with
the same constituent in the original rock. This can also readily
be accomplished by a process the formula for which is given
below, 2 and by which are obtained the results given in cloumns
V and VI.

1 G. Roth, Allegemeine u. Chemische Geologie, 3d ed.

2 The formula employed in these calculations is as follows : B ~ == x '

and 100 x = y, in which A = the percentage of any constituent in the
residual material ; B = the percentage of the same constituent in the fresh
rock, and C=-the quotient obtained by dividing the percentage amount of
alumina (or iron sesquioxide, whichever is taken as a constant factor) of
the residual material by that in the tresh rock, the final quotient being
multiplied by 100. x then equals the percentage of the original constituent
saved, in the residue, and y the percentage of the same constituent lost.

WEATHERING OF GRANITE

189

DISINTEGRATED AND" DECOMPOSED GRANITE, DISTRICT OF COLUMBIA, SHOWING
PROPORTIONAL Loss OF CONSTITUENTS

CONSTITUENTS

PERCENTAGB
Loss FOR EN-
TIRE ROCK

PERCENTAGE
OF EACH CON-
STITUENT SAVED

PERCENTAGE
OF EACH CON-
STITUENT LOST

Silica (SiO 2 )

10.50%

85.11%

14.89%

Alumina (A^Oj)

0.46

96.77

3.23

Iron sesquioxide (Fe 2 O 8 )
Iron protoxide (FeO)

} 0.00

100.00

0.00

Lime (CaO)

0.81

74.79

25.21

Magnesia (MgO) . . .

0.36

98.51

1.49

Soda (Na 2 O)

0.77

71.38

28.62

Potash (K 2 0)

0.85

68.02

31.98

Phosphoric anhydride (P 2 O 6 ) . . .
Ignition

0.04
2.16 1

60.00
100.00

40.00
0.00

Total loss

13 79 %

From a perusal of these figures, it appears that the residual
sand retains 85.11% of the original silica; 96.77% of the alu-
mina; all the ferric oxide; 74.79% of its lime; 98.51% of its
magnesia, together with 71.38% of its soda and 68.02 of the
potash, while there has been an actual gain, as was to be ex-
pected, in volatile matter.

Let it not be too hastily assumed that we have exhausted the
subject.

It must be remembered that while an analysis shows the actual
composition of a rock so far as the various elements are con-
cerned, it quite fails to show the manner in which those elements
are combined. While the ultimate composition of the fresh and
decomposed samples may be closely similar, it is possible, indeed
probable, that in some cases at least the manner of combination
of these elements is quite different. This is well illustrated in the
case of the figures showing the percentages of alumina in anal-
yses I and III and which differ only nine-tenths of one per cent
in total amount; yet in the first the alumina exists mainly in
the form of anhydrous silicates of alumina, potash, iron, and
magnesia (as in the feldspars and mica), while in the last a very
considerable proportion, or indeed all in extreme cases of weath-

1 Gain.

190

KOCK DISINTEGRATION AND DECOMPOSITION

ering, may exist as a hydrous silicate of alumina only (kaolin).
It is in instances of this kind that the microscope may render
efficient service, and much may be learned by means of such
mechanical analyses as can be made by sifting and washing.
Such separations made on this disintegrated rock showed it to
consist of particles as given in the following table, the 4.25%
silt being obtained by washing the 10.75% of material which
passed through fine bolting-cloth of 120 meshes to the lineal inch,
and which represents the impalpable mud remaining in sus-
pension while the 6.5%) of fine sand sank quickly to the bottom
of the beaker in which the washing was made. The residual
sand yielded then :
Silt 4.25% Largest grains 0.1 mm. in diameter

Very fine sand .... 6.50

Fine sand 11.25

Medium sand 3.80

11.00

23.50

Coarse sand 29.50

Gravel 10.20

Sand

0.18
0.25
0.65
1.00
1.50
2.00
8.00

Total 100.00%

The coarser of these particles, like the gravel and coarse sand,
are of a compound nature, aggregates of quartz and feldspar,
with small amounts of mica and other minerals. In the finer
material, on the other hand, each particle represents but a single
mineral, the process of disaggregation having quite freed it from
its associates, excepting of course, the microscopic inclusions
which could be liberated only by a complete disintegration of the
host itself. These particles, as seen under the microscope, are all
sharply angular, and in many cases surprisingly fresh, though
the analyses had suggested only a slight change in chemical com-
position. The mica shows the greatest amount of alteration, the
change consisting mainly in an oxidation of its ferruginous con-
stituent, whereby the folia becomes stained and reduced to yel-
lowish brown shreds. The feldspars are, in some cases, opaque
through kaolinization, but in others are still fresh and unchanged
even in the smallest particles. The finest silt, when treated with
a diluted acid to remove the iron stains, shows the remaining
granules of quartz, feldspar, and epidote beautifully fresh, and
with sharp, angular borders, the mica being, however, almost
completely decolorized.

WEATHEEING OF GKANITE

191

An analysis of the silt, which was found to constitute 4.25%
of the entire mass of disintegrated material, as noted above,
is given below, and also a partial separation and analysis of the
39.7% soluble, and 60.3% insoluble portions. 1

ANALYSES OF SILT FROM DISINTEGRATED GRANITE, DISTRICT OF COLUMBIA

CONSTITUENTS

III

BULK ANALYSIS
OF SILT

ANALYSIS OF
SOLUBLE PORTION
(39.7%) SILT

ANALYSIS OF
INSOLUBLE PORTION
(60.8 %) SILT

Ignition ......

8.12%
49.39 {

23.84
3.69
4.41 ^
4.60
3.36 f
2.49 J

8.12%
InHCl 1.123
In Na 2 CO 8 l 1.147
9.21
4.47

Not det.

0.97%
} 37.30

13.40
0.82
{2.90
Trace
2.75
1.07

Silica (SiO 2 )

Alumina (A1 2 3 ) ....
Iron sesquioxide (Fe 2 3 ) .
Lime (CaO)

Magnesia (MgO) ....
Soda (Na 2 O)

Potash (K 2 O)

99.90%

34.07

59.21

93.28 %

From these analyses it would appear that of the 17 grammes
of silt, representing 4% of the total disintegrated material,
only 39.7% is soluble; and, further, that a very considerable
proportion of the insoluble residue, as indicated by the high
percentages of alkalies and lime, still consist of unaltered soda-
lime and potash feldspars, the iron and magnesia alone having
been largely removed.

These results are not quite what one would be led to expect
from a perusal of the literature bearing upon the subject of
rock decomposition. As long since noted by J. G. Forch-
hammer, G. Bischof, T. Sterry Hunt, and others, the ordinary
processes of decay in siliceous rocks containing ferruginous
protoxides and alkalies consists in the higher oxidation and

1 In all analyses made by or under the direction of the author, the matter
tabulated as soluble is that extracted by boiling for three hours in hydro-
chloric acid of one-half normal strength, to which is added the silica set
free in a gelatinous form by the acid and subsequently extracted by sodium
carbonate solution. All analyses made on material first dried at 100 C.

192 BOCK DISINTEGRATION AND DECOMPOSITION

separation of the protoxides in the form of hydrous sesqui-
oxides and a general hydration of the alkaline silicates, accom-
panied by the formation of alkaline carbonates, which, being
readily soluble, are taken away nearly as fast as formed. More
or less silica is also removed, according to the amount of car-
bonic acid present, a portion of the alkalies forming soluble
alkaline silicates when the supply of the acid is insufficient to
take them all up in the form of carbonates. The apparent
anomaly here shown is partially explained by examination of
the various separations with the microscope. Thus the low
percentage of silica is found to be in large part due to the fact
that the residual quartz granules are, in many cases, too large
to pass the 120-mesh sieve, or, if passing, have been largely
separated in the process of washing. Further, it is found that
the sifting has served to concentrate the small epidotes in the
fine sand, and a portion of them have even come over with
the silt The presence of this epidote also explains in part the
high percentage of lime shown, since the mineral itself carries
some 20 to 24% of this material The large percentages of
magnesia, soda, and potash cannot, however, be thus accounted
for, and we are led to infer that the feldspathic constituents, to
which the alkalies are to be originally referred, have undergone
a mechanical splitting up rather than a chemical decomposition.
This view is, to a certain extent, borne out by microscopic studies,
but it is difficult to measure by the eye the relative abundance
of these constituents with sufficient accuracy to enable one to
form a satisfactory conclusion. The magnesia must come from
the shreds of mica, many of which, from their small size and
almost flocculent nature when decomposed, would naturally be
found in the silt obtained as stated.

It is to be noted that the magnesia, together with the iron,
exists almost wholly in a soluble form.

It is evident at once that we have had to do here with
but the preliminary stages of granitic weathering, that the
process is more one of disintegration than decomposition, and
it will be well to consider now a case in which the decom-
position has gone on to the condition of a residual clay, as
found in many of the Southern states. For this purpose a
biotite gneiss or gneissoid granite fround near North Garden,
in Albemarle County, Virginia, is selected. The temperature
average is here about 56.5 Fahr., with recorded extremes of

WEATHEKING OF GRANITE 193

-12 and +1)7 Fahr. The annual precipitation is 48.88
inches. The rock is a coarse gray feldspar-rich variety with
abundant folia of black mica. Under the microscope it shows
the presence of both potash and soda-lime feldspars, a sprinkling
of apatite and iron ores, sporadic occurrences of an undetermined
zeolite, and an extraordinary number of minute zircons which
are mostly enclosed in the feldspars. There are also present
occasional small garnets and aggregates of decomposition prod-
ucts the exact nature of which was not made out. The residual
soil resulting from the decomposition of this rock is highly plas-
tic, of a deep red-brown color, and has a distinct gritty feeling
in the hand, owing to the presence of quartz and undecomposed
silicate minerals. In columns I and III on the next page are given
the results of analyses of fresh rock and residual soil, and in II,
IV, and V the analyses of the soluble and insoluble portions. In
columns VI, VII, and VIII are given the calculated percentage
amounts of the various constituents saved and lost, as before.

The particular features to which attention need be called,
are (1) that 30.51% of the fresh rock and 69.18% of the
decomposed are soluble in hydrochloric acid and sodium car-
bonate solutions, and that more than half the potash and
nearly the same proportion of the soda in the fresh rock is
found in the acid extract. (2) That the insoluble portion of
the residuary material is mainly in the form of free quartz.
(3) That 44.67% of the original matter has been leached away,
and that (4) of the original silica 52.45% is lost, while 85.61%
of the iron and all the alumina remain. All the lime has dis-
appeared, 83.52% of the potash, 95.03% of the soda, and 74.70%
of the magnesia. The total amount of water, as indicated by
the ignition, has increased very greatly, as was to be expected.
The small original amount of phosphoric acid prohibits our
placing too much reliance upon the indicated gain in this con-
stituent, since it may be due to errors in manipulation.

To guard against the danger of making deductions from in-
sufficient data, another fairly typical example may be selected
this time a dark blue-gray rock of medium, massive texture from
near Greenville, Georgia, as analyzed and described by Dr.
Thomas Watson. 1 The rock in its fresh condition consists of
quartz, the potash feldspars orthoclase and microcline, a soda-
lime feldspar near oligoclase, biotite, a little muscovite, and the

1 Granites and Gneisses of Georgia, p. 313.
14

194 ROCK DISINTEGRATION AND DECOMPOSITION

5"- tH <H

T* O CO O I O O O O

jo aStrjnaoaaj

6801

uotiJoj oiqnjos

[Oil
-JO<I JO SIS^IBHV

Biscay 3una

rH O 1 CO

t-* o o d -o o

M< O 00 C--J rH

80Or^OOCOOO
OCOrtHCOOOOO

00 r-,

'. CO*

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9{8^av

COO ** Tf<OiOOCO<M
rHQO CO rHO^rHOd

WEATHERING OF GNEISS

196

usual scattering of apatite and zircon. The quartz is penetrated
by rutile needles and the potash feldspars predominate.

Decay has progressed to an approximate depth of 20 feet, the
upper ten or fifteen feet being reduced to the condition of a
deep red clay, with a gritty feeling, owing to the presence of
particles of quartz. Abundant shreds of bleached biotite were
distinctly visible to the unaided eye. At greater depths the
dark-red clay passes through lighter and brighter red phases, to
slightly reddish-gray and fairly firm rock. The materials for
analyses were taken from a distance of 5J feet and 35 feet below
the surface. The results obtained were as below :

ANALYSES OF FRESH AND DECOMPOSED BIOTITE GRANITE FROM GREENVILLE,
MERIWETHER COUNTY, GEORGIA

Calculations Based on Decomposed
Rock 5% Feet Below Surface

X 3

Recalculated on a

CONSTITUENTS

Basis of 100

jgj

**!

I 1 !

SiO,
ALA

69.88
16.42

51.29
2969

69.28
16.28

51.03
29.54

53.48
7.13

22.80
56.18

77.20

43.82

FeA 1

1.96

6.33

1.95

6.30

0.00

100.00

0.00

CaO

1.78

0.07

1.77

0.07

1.74

1.22

98.78

MgO

0.36

0.14

0.36

0.14

0.31

12.06

87.94

Na,O

4.46

1.12

4.42

1 12

4.07

7.84

92.16

K 2

5.63

1.50

5.58

1.49

5.11

8.25

91.75

Ignition.

0.36

10.36

0.36

10.31

0.00

100.00

0.00

Total.

100.85

100.50

100.00

71.74

From this it appears that 71.84% of the original rock material
has disappeared, the principal constituents being lost in the
following proportions: Silica, 77.20%; alumina, 43.82%, lime,
98.78% ; magnesia, 87.94% ; soda, 92.16% ; and potash, 91.75%.
Analyses of the same material from a depth of ten feet below the
surface showed a total loss of 61.98%, indicating a less advanced
stage of decomposition, as was to be expected.

The region is one in which the recorded extremes of tempera-
1 All iron was estimated as F 2 O 3 . The iron assumed to be constant

196

BOCK DISINTEGRATION AND DECOMPOSITION

ture are 8 and -{- 3.00 Fahr., with an average annual tem-
perature of 62.1 and a rainfall of 50.38 inches. 1

Weathering of Nepheline Syenite. Passing from the most
acid group of granular crystalline rocks, we will next consider
the elaeolite syenites of the Fourche Mountain region of Arkansas.
The Weather Bureau records for the region show a maximum
range of temperatures amounting to 118, with an annual aver-
age of 61.5 and an annual precipitation of 53.63 inches. The
rocks are somewhat coarsely crystalline granitic-appearing, with
an orthoclase feldspar in broadly tabular forms as the prevailing
constituent, though always accompanied by nepheline, biotite,
pyroxene, titanite, and apatite, while fluorite, analcite, and
thomsonite, together with calcite, occur as secondary products.
The rock weathers away to a coarse gray gravel which ultimately
becomes a clay, from which, by washing, may be obtained kaolin
in a fair degree of purity.

ANALYSES OF FRESH AND DECOMPOSED SYENITE, ARKANSAS

FRESH

SYENITK

DECOMPOSED SYENITE

KAOLIN-LIKE
RESIDUE

CONSTITUENTS

Silica (SiO a ) ....

69.70%

68.50%

50.65%

46.27 %

Alumina (A1 2 3 ) . .

18.85

25.71

26.71

38.57

Ferric oxide (Fe 2 3 ) .

4.85

3.74

4.87

1.36

Lime(CaO) ....

1.34

0.44

0.62

0.34

Magnesia (MgO) . . .

0.68

Trace

0.21

0.25

Potash (K 2 O) ....

6.97

1.96

1.91

0.23

Soda (Na 2 0) ....

6.29

1.37

0.62

0.37

Ignition (H 2 0) . . .

1.88

5.85

8.68

13.61

99.56%

97.57 %

94.27 %

101.00%

1( The following table shows the total losses calculated from a number of
analyses of more or less decomposed granites from Georgia, as given by
the authority quoted above:

Biotite Granite (partially decomposed), near Elberton 7.92

Oglesby 7.71

Lexington 14.56

Appling 15.84

Lithonia 26.69

Camak 34.04

Coweta Station . . . 35.07

Newman 38.45

Oglesby 44.72

Greenville . 71.84

WEATHERING OF SYENITE AND PHONOLITE

197

The analyses on the preceding page from the work of Dr. J. F.
Williams 1 will serve to show the changes which have here taken
place in the transformation from (I) fresh syenite through (II
and III) intermediate stages of decomposition to (IV) a kaolin-
like residue.

Kecalculating the numbers given in columns I and IV upon
the basis of 100, we obtain by further calculations the figures
given in columns V and VI and VII below, which represent the
proportional loss of each constituent, as before.

CALCULATED Loss OF MATERIAL

VII

CONSTITUENTS

PERCENTAGE
Loss FOR ENTIRE
BOCK

PERCENTAGE
OF EACH CON-
STITUENT SAVED

PERCENTAGE
OF EACH CON-
STITUENT LOST

Silica (Si0 2 )

37. 28% loss

37.82 %

62.18%

Alumina (Al 2 0a)

0.00

100.00

0.00

Ferric oxide (Fe 2 0s)

4.19

13.83

86.17

Lime fCaO)

1.19

12.10

87.90

Magnesia (MgO)

0.57

17.90

82.10

Potash (K 2 O)

6.90

18.15

81.85

Soda (Na 2 O)

6.15

2.89

97.11

Water (H 2 0) . . .

0.00

100.00

0.00

Total loss of original material.

65.28#

Here, as with the granitic rocks, it will be noted there is a
gradual increase in the percentage of water as the decomposi-
tion advances, and a decrease in the amount of silica even more
pronounced. This last, as may be readily imagined, is due to
the absence of free quartz in the Fourche Mountain rocks.

Weathering of Phonolite. The phonolites of Marienf els, near
Assig, in Bohemia, have been described by Lemberg 3 as weather-
ing into a bright-colored, porous, friable mass, the composition
of which, as compared with the fresh rock, is shown on the next
page. Each column, it should be stated, represents an average
of three analyses, I being the fresh and II the weathered ma-
terial, while in III, IV, and V are given the percentage calcula-
tions of gain and loss, as before.

1 Ann. Eep., Vol. II, 1890, Arkansas Geol. Survey.

*Zeit. der Deutschen Geol. Gesellschaft, Vol. 35, 1883, p. 559.

198 ROCK DISINTEGRATION AND DECOMPOSITION
ANALYSES OP FRESH AND DECOMPOSED PHONOLITE, BOHEMIA

CONSTITtTENTS

FRESH
PHONOLITE

DECOMPOSED
PHONOLITE

Loss OF
CONSTITUENTS

PERCENTAGE
OF EACH
CONSTITUENT

SAVED

PERCENTAGE
OF EACH
CONSTITUENT

LOST

Silica (SiO 2 )

55.67 %

55.72%

4.83 %

91.46%

8.54%

Alumina (Al 2 Os)

20.64

22.19

98 40

1 60

Ferric oxide (Fe 2 0a)

3.14

3.44

0.00

100.00

100 00

Lime (CaO)

1.40

1.28

0.25

83.66

16 34

Magnesia (MgO)

042

044

95 65

4 35

Potash (K 2 O) . .

5 56

6 26

OO 1

100 00

Soda (Na 2 O) . . . .

7.12

2.65

4 79

34 01

65 99

Ignition .........

4.33

7.79

0.00 !

100 00

98.28%

99.77%

10.26%

....

This phone-lite, it should be remarked, consisted essentially
of sanidin feldspars and a soda zeolite, together with accessory
augite, black mica, magnetic and titanic iron, and possibly
hauyne. The zeolite is assumed to have originated from the al-
teration of the nepheline. The process of decomposition would
seem to consist, then, in the breaking down of this zeolite, and
the conversion of the rock into an earthy mass, with little other
change, so far as ultimate composition is concerned, than a loss
of a considerable proportion of its soda, and an assumption of
nearly 3.5% of water. The decomposed rock yielded 55.44%
of material insoluble in hydrochloric acid, with essentially
the composition of sanidin, showing that this mineral underwent
only a physical disintegration, the decomposition proper being
limited to the other constituents. 2

Weathering of Diabase. Turning to still more basic rocks,
we will next consider a disintegrated diabase occurring in the
form of a large dike extending from Granite Street in Somerville,

1 Gain. The calculations for potash in column IV gives: 107.79% and for
ignition 164.77%.

2 In calculating these analyses, it was found that the loss of alumina had
exceeded that of iron oxide, necessitating the assumption of the last-named
as a constant for comparison. The apparent gain in potash is presumably
due to errros in analysis, since, as will be noted, the analysis of the fresh
material, given in column I, foots up only

WEATHERING OF DIABASE 199

Massachusetts, to Spot Pond in Stoneham, and beyond. 1 The
average annual temperature for the region is 48.60 Fahr., with
recorded extremes of 13 and + 102. The ground remains
frozen and often covered with snow for at least four months of
the year. The annual precipitation is 44.96 inches. The rock
at the point selected for study (Medford) is a coarsely granu-
lar admixture of lath-shaped feldspar, black mica, augite, and
brown basaltic hornblende, with the usual sprinkling of apatite,
magnetite, and ilmenite. Secondary uralite, chlorite, biotite,
leucoxene, kaolin, calcite, pyrite, and quartz are common. 8

The rock has undergone extensive disintegration, giving rise
to loose sand and gravel of a deep brown color, in which lie
rounded boulders of all sizes of the still undecomposed material.
These boulders, as is usually the case, show a more or less con-
centric structure, from without inward, until a solid core of
unaltered diabase is met with. (See PI. 17, and Fig. 2, PL 22.)

A mechanical separation of the disintegrated material yielded
results as below :

1. Coarse gravel above 2 mm. in diameter 42.300%

2. Fine gravel " 2-1 mm. in diameter 20.355

3. Coarse sand " 1-5 mm. in diameter 12.723

4. Medium sand " .5-.2S mm. in diameter 9.567

5. Fine sand " .25-.! mm. in diameter 4.907

6. Very fine sand " .1-.05 mm. in diameter 4.181

7. Silt " .05-.01 mm. in diameter 1.128

8. Fine silt " .01-.005 mm. in diameter 0.370

9. Clay " .005-.0001 mm. in diameter 1.670

10. Loss at 110 C. . 0.660

11. Loss on ignition 1.730

99.691%

Of the above, the first three sizes could be easily recognized
by the unaided eyes, as composed of particles of a compound
nature. In number 4 the separation had gone a trifle farther,
though even here inspection with a pocket lens revealed the
compound nature of many of the granules, somewhat obscured
by the prevailing discoloration from the oxides of iron. It

*See Disintegration and Decomposition of Diabase at Medford, Massa-
chusetts, by G. P. Merrill, Bull. Geol. Soc. of America, Vol. VII, 1896, pp.
349-362.

2 On the Petrographic Characters of a Dike of Diabase in the Boston
Basin, by W. H. Hobbs, Bull. Mus. Comp. Zoology, Vol. XVI, No. 1, 1888.

200

BOOK DISINTEGBATION AND DECOMPOSITION

forms a gray-brown sand composed of feldspathic
dirty brown augites, and lustrous scales of brown mica. Num-
bers 5 and 6 seemed composed almost wholly of beautifully
lustrous, dark mahogany-brown mica scales, while 7 would pass
for a finely micaceous umber. Numbers 8 and 9 were uni-
formly ochreous and without appreciable grit.

ANALYSES OF FRESH AND DISINTEGRATED DIABASE FROM MEDFORD

FRESH DIABASE

DISINTEGRATED
DIABASE

SILT FROM DISINTEGRATED
DIABASE, Nos. 7, 8 AND 9
OF TABLE, ON P. 199.

III

VII

CoHVnnrKirra

kag

!,,

00 D ff

**"!

Si* 5

"& ^5

"a ^

CB ^

O i i "^

.2 -3 T3

f||

ill

^ g O

1|5

1 o 5

J jf 5

u -c w

^ o.M

-< o<5

SiO 2 ....

47.28

44.44

1351

36.61

f Sol. in HC1 .

1.19

0.85

0.47

I Sol. inNa^Coj

9.66

8.65

22.63

AIA ....

20.22

4.74

23.19

4.86

21.98

)

Fe 2 3 ....
FeO . . . .

3.66
8.89

} 10.91

12.70

10.00

12.83

Y 5.88

40.68

CaO ....

7.09

3.09

6.03

1.50

3.32

0.12

3.44

MgO . . . .

3.17

2.20

2.82

1.84

3.23

0.79

4.02

MnO ....

0.77

Not det.

0.52

Not det.

K 2 O .

2.16

1.21

1.75

0.68

1.30

0.52

1.82

Na 2 O ....

3.94

0.50

3.93

0.17

0.90

1.24

2.14

PA ....

0.68

Not det.

0.70

Not det.

Ignition .

2.73

3.73

10.86

0.11

10.97

100.59

36.23

99.81

32.28

77.52

22.17

99.68

The chemical nature of the fresh and decomposed rock is
shown in the accompanying table, the results being in nearly
every case averages obtained from two or more analyses.
The "fresh" material, obtained from the interior of one of the
boulders, is firm in texture, has a bright clean fracture, and
shows to the unaided eye no signs of decomposition. When pul-
verized and treated with acid, however, it effervesces distinctly,

WEATHERING OF DIABASE 201

indicating the presence of free carbonates, which are also observ-
able as secondary calcite when thin sections are examined under
the microscope. Some of this calcite is evidently a deposit from
infiltrated waters, being derived from the surrounding decom-
posed material, while a portion results from the decomposition
of the silicate minerals in place. Aside from a slight kaolini-
zation of the feldspars and development of chlorite from the
ferruginous silicates, there are no other observable signs of de-
composition, though the presence of a soda-bearing zeolite is indi-
cated by cubes of chloride of sodium, which separate out when
an uncovered slide is treated with a drop of hydrochloric acid.

A glance at this table is sufficient to show that the disinte-
gration is accompanied by decomposition and a leaching action
which has resulted in the removal of a portion of the more
soluble constituents. The fact that the fresh rock yields the
larger percentages of its constituents to the solvent action of
acid and alkaline solutions is readily explained on this ground,
though it may be doubted if the full significance of the fact, so
far as it relates to siliceous crystallines, is as yet appreciated.
It will be observed that 36.23% of the fresh rock and 32.28%
of the decomposed is thus extracted.

Of the material classed as silt in columns V, VI, and VII, or
as silt and clay, on p. 199, and which constitutes only some
3.17% of the entire residual debris, 77.87% is soluble in dilute
hydrochloric and sodium carbonate solutions. The insoluble
portion, constituting 22.13% of the silt, consists of unaltered
feldspar and iron, lime and magnesian silicates, which are easily
recognized under the microscope, in the form of minute, sharply
angular particles. Recalculating, as before, the matter in col-
umns I and II on the basis of 100 and considering the alumina
as a constant factor, we obtain the results given in columns VIII
to XII inclusive, on p. 202, representing, so far as it can be ob-
tained by this method, the actual percentage loss of materials
attending the breaking down.

From the figures in column X it appears that there has
been a loss of some 14.93% of all constituents. The increase
in water, as indicated by the ignition, is a natural consequence
of hy drat ion and the presence of a small amount of organic
matter. This increase, it should be stated, is greater than may
be at first apparent, for the reason that the fresh rock contains
a considerable amount of secondary calcite, which is quite lack-

202

EOCK DISINTEGRATION AND DECOMPOSITION

ing in the residual sand. A large part of the ignition in col-
umns I and VIII is therefore to be accredited to carbonic acid,
and not to water of hydration.

CALCULATED Loss OF MATERIAL IN MEDFORD DIABASE.

VIII

XII

CONSTITUENTS

KECALCU
BASIS

LATED ON
OF 100

il.

Fresh
Diabase

Decomposed
Diabase

*f 3
11

jUj

li\

}|i

Silica (SiOg)

47.01 %

44.51 %

8.48

81.97%

18.03%

Alumina (A1 2 O 8 ) . . .
Ferric oxide (FegOg) .
Ferrous oxide (FeO) . .
Lime (CaO)

20.11
3.63

8.83
7.06

23.24
1 12.71
6.04

0.00
2.42
1.83

100.00
81.90
74.11

0.00
18.10
25.89

Magnesia (MgO) . . .
Manganese (MnO) . .
Potash (K 2 0) ....
Soda (Na 2 0) .

3.15
0.77
2.14
3 91

2.85
0.52
1.75
3.94

0.68
0.32
0.62
0.50

78.30
58.43
70.85
87.17

21.70
41.57
29.15
12.83

Phosphoric acid (P 2 5 ) .
Ignition ..... .

0.68
2.71

0.70
3.74

0.08
0.00

88.61
100.00

11.39
0.00

100.00%

100.00 %

14.93%

....

From columns XI and XII it appears that of all the essential
constituents, the lime and potash salts have suffered the most,
though the iron oxides have been carried away to the amount
of 18.10%. Magnesia has also proven very susceptible to the
solvent action, disappearing to the amount of 21.70% ; and
lastly, silica, to the amount of 18.03%. The small original
amounts of manganese and phosphoric acid render the results
obtained by these calculations of doubtful value, since it is pos-
sible they may be due to errors of analysis.

In this case, as in that of the granite from the District
of Columbia, we have to do with only the earlier stages of de-
generation, with conditions which are as much in the nature
of mechanical disintegration as of chemical decomposition. As
before, then, it will be instructive to consider cases in which, in
rocks of similar nature, the decomposition has proceeded much
farther. For this purpose we will select a diabase from near

\VEATHERING o*' DIABASE

203

Chatham, Virginia, analyzed and described by Dr. Thomas L.
Watson. 1 The rock in its fresh state is dark-gray, homogeneous,
of medium texture, showing to the naked eye feldspars and
augites, but under the microscope an abundant sprinkling of
olivine and magnetite, some biotite and secondary serpentine and
chlorite. The feldspar was shown by analyses to be labradorite.
The rock in weathering breaks down into the usual boulder
masses, the transition from the bright orange residual clay to the
hard fresh rock being quite sharp, so that it is possible to ob-
tain hand specimens showing within the space of a few inches
all stages of the process.

The material analyzed, as given on p. 204, was taken from the
outer portion of a small boulder such as were scattered through-
out the mass of residual incoherent clay, and concerning the
origin and derivation of which there could be no question. This,
although of a nature to be called, on casual inspection, an ochre,
showed on close examination a spongy mass of iron sesquioxide
through which was distributed a perfect network of white kao-
linized masses of the original feldspar. To the unaided eye the
mass seemed thoroughly decomposed without any trace of the
original silicate minerals preserved, but after removing the iron
oxide by continued digestions with very dilute hydrochloric acid,
and the residue subsequently examined under the microscope,
considerable traces of both undecomposed feldspar and augite
were distinctly recognizable, with surprisingly large quantities
of magnetite. Apparently, the magnetite was in as fresh con-
dition and in as large quantities as in the fresh and unaltered
rock. A mechanical analysis yielded the results given below :

DIAMETER IN MM.

CONVENTIONAL NAMES

PER CENT.

(1) 2-.1

Fine gravel.

0.00

(2) 1-.5

Coarse sand.

3.21

(3) .5-.2S

Medium sand.

13.10

(4) .25-.!

Fine sand.

15.39

(5) .1-.05

Very fine sand.

23.49

(6) .05-.01

Silt.

23.98

(7) .01-.005

Fine silt.

4.16

(8) .005-.0001

Clay.

14.20

Total

97.53

American Geologist, Vol. XII, 1898, p. 85.

204

BOOK DISINTEGEATION AND DECOMPOSITION

Nos. 2, 3, and 4 show clearly to the unaided eye the com-
pound character of the mass, and when examined under the
microscope, distinct particles of the undecomposed silicate min-
erals with an abundance of magnetite are to be seen. In No. 5
the decayed products become more differentiated into kaolin and
iron oxide, with the usual amount of magnetite and some of the
undecomposed silicate minerals still discernible. Nos. 6 and 7
represent highly colored ferruginous masses in which a slight
sprinkling of the feldspar, augite and magnetite can still be
recognized, though to a very much less degree in No. 7 than in
No. 6. No. 8 is a deep buff-colored mass of fine clay showing no
trace of the original minerals whatsoever.

According to the analyses and calculations given below,
there has been a total loss amounting to 70.31% or more than
two-thirds of the original material. This includes a loss of
73.64% of the original silica, 68.19% of the alumina, 98.68%
of the lime, 98.81% of the magnesia, 82.46% of the soda, and
77.31% of the potash. 44.59% of the fresh rock was soluble
in dilute hydrochloric acid and sodium carbonate solutions, and

ANALYSES OF FRESH AND DECOMPOSED OLIVINE DIABASE, FROM CHATHAM,

VIRGINIA

FRESH

DECOMPOSED

CALCULATED

CALCULATED AMOUNTS

DIABASE

TO TOTAL

SAVED AND LOST

or 100

III

VII

VIII

%__

o .S<

.2(5

SD<D

j?02

ill

Sfl-d

G O 2

ill

III

SiO,

45.73

12.78

37.09

12.72

45.38

36.77

33.41

26.37

73.64

A1A

13.48

9.10

13.19

8.22

13.38

13.08

9.11

31.81

68.19

Fe 2 O

11.60

8.75

35.69

28.25

11.51

35.39

0.00

100.00

0.00

CaO

9.92

4.37

0.41

0.02

9.84

0.41

9.71

1.32

98.68

MgO

15.40

8.65

0.57

0.20

15.28

0.56

15.09

1.19

98.81

Na 2 O

3.24

Not det.

1.75

1.00

3.21

1.73

2.64

17.54

82.46

K 2 O

0.47

0.33

0.20

0.47

0.33

0.35

22.69

77.31

H 2 O

0.94

11.83

0.93

11.73

0.00

100.00

0.00

Total

100.78

44.59

100.86

62.44

100.00

70.31

Gain.

WEATHERING OF BASALT

205

62.44% of the decomposed. The large percentage of soluble mat-
ter in the last case, it should be noted, is due mainly to the
iron oxide. The region is one of a mean average temperature
of 56.9 Fahr., and recorded extremes of 6 and +102,
with a rainfall of 42.85 inches. The ground, as a rule, freezes
in winter to but a slight depth, and remains so but a few days
at a time.

Weathering of Basalts. Eesearches on basalts in Bohemia
and France yielded Ebelman 1 results of similar import, although
in neither case had decomposition gone so far as in that de-
scribed by Dr. Watson.

In the case of the Bohemian basalt, the decomposition com-
menced with the formation of boulders, which, when the
process had not gone too far, still showed fresh, unchanged

ANALYSES OF FRESH AND DECOMPOSED BASALT FROM BOHEMIA

III

i w

ff>

o ^

CONSTITUENTS

P o

g S

S o

5 s

4 B M

H ^

^ s s

^ 02

3 H

1? ^

S5 O ^

s g

g g g

H O
02

cS g

(Sw "

w g

Silica (Si0 2 ) ....

43.61

43.00

43.27

15.04 loss

67.01

32.99

Alumina (A1 2 O 3 ) . .

12.26

13.90

18.13

0.00 "

100.00

0.00

Ferric iron (Fe 2 O 8 ) . .
Ferrous iron (FeO) .

3.51
12.16

5.40\
8.30J

11.70

9.10 "

49.83

50.17

Lime (CaO) ....

11.37

12.10

2.60

9.60 "

54.47

84.53

Magnesia (MgO) . . .

9.14

7.30

3.40

6.83 *

25.90

74.10

Soda (Na 2 O) ....
Potash (K 2 0) ....

2.72"!
0.81 J

0.50

0.20

3.39

38.31

61.69

Water (H 2 0) ....

4.42

9.50

20.70

0.00

100.00

....

100.00 %

43.96

....

basalt interiorly, but became more and more altered toward
their peripheries. The first stage of decomposition (column II),
it will be noted, consists, aside from hydration, in a slight appar-
ent loss of silica, a considerable oxidation of the iron magnesia
minerals, accompanied by a slight loss of both constituents, and

Ann. des Mines, Vol. VII, 1845.

206

KOCK DISINTEGRATION AND DECOMPOSITION

an almost complete loss of alkalies. In the second stage (column
III) lime and magnesia are both lost in considerable amounts,
the iron passing over wholly to the condition of sesquioxide, and
there is a further slight diminution in the proportional amount
of silica. It is evident that here the feldspars were the first of
the constituents to yield to the decomposing forces, the augite
and olivine proving most refractory. The total loss of material,
it will be noted, amounts to 43.96%, the lime, magnesia, alka-
lies, iron oxides, and silica disappearing in the order here
mentioned.

In the case of the basalt from Crouzet, the analyses show a
total loss of 60.12%, or over one-half of the original material.
This loss includes nearly two-thirds of the original silica,

ANALYSES OF FRESH AND DECOMPOSED BASALT FROM THE HAUTE LOIRE,

FRANCE

III

CONSTITUENTS

SBS

< * s

* % g

|A M

Ili

8 S

* S3

fiw g

w 5

Silica (Si0 2 ) ....

48.29 %

37.09%

30.34% loss

34.44 %

65.56%

Alumina (A1 2 O 3 ) . . .

13.25

30.75

0.00 "

100.00

0.00

Ferric iron (Fe 2 O 3 ) . .
Ferrous iron (FeO) .

0.00
16.66

4.311
O.OOJ

16.64

11.16

88.84

Lime (CaO) ....

7.33

8.97

3.46

52.76

47.24

Magnesia (MgO) . . .

7.03

0.61

6.77 "

3.62

96.38

Potash (K 2 O) ....

1.81

0.71

1.51 "

16.66

83.34

Soda (Na 2 O) ....

2.71

1.01

1.40 "

25.59

74.41

Ignition .

4 92

16 55

100.00

0.00

100.00 %

60.12%

....

88.84% of the iron, and 96.38% of the magnesia. The loss
of both iron and magnesia in such proportionally large quan-
tities is quite unusual, and indicates, so far as the iron is con-
cerned, that the decomposition took place under conditions
excluding a sufficient supply of oxygen to convert the same
into the insoluble sesquioxide, or where subjected to the de-

WEATHERING OF DIOKITE

207

oxidizing and solvent action of organic acids. The removal of
the magnesia, which must have existed mainly in the mineral
olivine, indicates that the decomposition has gone on even to
the production of carbonate of magnesia and the separation of
free silica and iron oxides.

Weathering of Diorite. An analysis by the present writer
of a closely related rock, a diorite, and its residual soil, from
North Garden, Albemarle County, Virginia, yielded the results
given in columns I and II below. The rock here was fine-
grained, of an almost coal-black color finely speckled with
whitish flecks due to the presence of feldspars. The microscope
showed it to be composed mainly of hornblende with interstitial
soda-lime feldspars and scattering areas of titanic iron. The
clay, or soil, to which it gave rise was deep brownish red in
color and highly plastic, though distinctly gritty from the pres-
ence of undecomposed minerals. In columns III, IV, and V are
given the loss and gain of the various constituents calculated
on an alumina constant basis.

ANALYSES OF FRESH AND DECOMPOSED DIORITE FROM ALBEMARLE COUNTY,

VIRGINIA

III

CONSTITUENTS

FRESH

DlORlTK

DECOM-
POSED
DIORITE

CALCULATED
Loss FOR EN-
TIRE ROCK

PER CENT
OF EACH
CONSTITU-
ENT SAVED

PER CENT
OF EACH

CONSTITU-
ENT LOST

Silica (Si0 2 )

46.75%

42.44%

17. 43% loss

62 69 L

37.31 /

Alumina (A1 2 8 ) . . .
Iron sesquioxide (Fe 2 O 3 ) l
Lime (CaO)

17.61
16.79
9 46

25.51
19.20
37

0.00 "
3.53
9 20

100.00
78.97
2 70

0.00
21.03
97 30

Magnesia (MgO) . . .
Potash (K 2 O) ....
Soda (Na 2 O)

5.12
0.55
2 56

0.21
0.49
56

4.97
0.21
2 17

2.83
61.25
15 13

97.17

38.75
84 87

Phosphoric acid (P 2 0) 5 .
Ignition

0.25
0.92

0.29
10.92

0.00
0.00

80.11
100.00

19.87
0.00

100.01%

99.99%

37. 51% loss

....

Weathering of Andesites. Results largely confirmatory of
these, but showing at the same time some interesting variations,
were obtained by J. B. Harrison from a study of the rocks and

*A11 iron calculated as Fe 2 O 8 -

208

EOCK DISINTEGBATION AND DECOMPOSITION

soils of Grenada/ the most southerly of the Windward Islands.
The entire island is volcanic, beyond the limits of the glacial
drift, and offers excellent opportunities for studies of this
nature, the rocks (mainly aiidesites and basalts) being decom-
posed to depths of over one hundred feet. Analysis of horn-
blende- and hornblende-augite andesites and soil derived through
their degeneration, yielded Dr. Harrison as below. 2

ANALYSES OF FRESH AND DECOMPOSED ANDESITES FROM GRENADA

3*1

H ,

8 O
O. (ft

wgg

|1|

IglJ

gS|

!I E

W{3 5

Silica (SiO 2 ) . . .

62.74

33.98

48.69

22.37

77.63

Alumina (A1 2 O 3 )

13.67

24.89

3.38

75.13

24.87

Ferric Iron (Fe 2 O 3 )
Ferrous Iron (FeO)

3.39
4.35

16.981
2.64 f

o.oo j

lOO.Oo}

0.00

Manganese (MnO) .

0.42

0.28

0.30

27.45

72.55

Lime (CaO) . . .

6.01

2.48

4.99

17.05

82.95

Magnesia (MgO)

1.74

2.28

0.80

54.15

45.85

Potash (K 2 O) . .

1.23

0.25

1.12

8.42

91.58

Soda (Na 2 O) . . .

4.25

1.35

3.69

13.13

86.87

Phosphoric Acid (P 2 O 6 )

0.18

0.14

0.12

31.82

68.18

Ignition . . .

2.02

14.55

0.00

100.00

0.00

100.00

99. 82 3

63.09

Perhaps the most striking feature of these calculations is the
unusually large percentage of silica lost. Alumina has also
been carried away in quantities above the average. The total
loss, amounting to 63.09% of the entire rock, is not in excess of
that noted in other cases, but doubtless approaches the maximum
amount, the available analyses showing a larger percentage in
but a single instance, that of the Chatham, Virginia, diabase
just noted.

Weathering of Ultra Basic Rocks. The ultra basic rocks, -
peridotites and pyroxenites, from the very nature of their
composition, must yield on decomposition residues poor in the

1 The Eocks and Soils of Grenada and Carriacou, and the Agricultural
Chemistry of Cacao, by J. B. Harrison, London, 1896.

2 The present writer has taken the liberty of recalculating these analyses
on the assumption that the ferric iron remained constant, rather than the
alumina, as assumed by Mr. Harrison.

8 The original analysis gives also CO 2 0.146%, SO 3 0.038%.

WEATHEKING OF ULTRA BASIC ROCKS 209

alkalies and rich in iron or aluminum k and magnesian com-
pounds. Owing, further, to their poverty in alkali-bearing
silicates, the process of decomposition must be less complex,
consisting essentially in hydration, oxidation, and a produc-
tion of iron, lime, and magnesian carbonates and a liberation
of chalcedonic silica.

During the process these rocks as a rule become brownish,
and, on the surface, often irregularly checked with a fine net-
work of rifts which become filled with secondary calcite, mag-
nesite, and chalcedony.

The deep green serpentines of Harford County, Maryland,
weather slowly down into a gray-brown soil, which consists of
60.17% silica, 10.40% of the iron oxides, 14.81% of alumina, and
only 7.23% magnesia. The fresh rock, on the other hand, car-
ries nearly 40% of magnesia, 8.50% iron and other metallic
oxides, and less than one-half of one per cent of alumina.

Natural joint blocks occur in which the preliminary stages
of weathering are manifested by a brown, ferruginous, though
tough and hard, vesicular crust of from a millimetre to two or
more centimetres' thickness, enclosing the slightly hydrated but
otherwise unchanged material.

An interesting case occurring near Manheim, in Herkimer
County, New York, of decomposition among igneous rocks of a
very basic nature and containing serpentine as a result of al-
teration from olivine has been described by Professor C. H.
Smyth, Jr., 1 and may be well referred to in detail. The original
rock consisted essentially of olivine, biotite, and probably melilite,
with accessory magnetite, apatite, and perofskite. Through
alteration the olivine had gone over into serpentine, as above
noted, and even in the freshest samples obtainable there was
some secondary calcite. In its unweathered state the rock, which
occurs in a dike some 26 to 30 inches in width in Calciferous
sand rock, is compact, dark-gray to nearly black in color, with
only a dark-brown mica conspicuous to the unaided eye. The
weathered portion is of a light yellowish-brown color and so
thoroughly disintegrated that the material can be easily scooped
out with the hand.

The material selected for analysis was taken at a depth of four

'Bull. Geol. Soc. of Am., Vol. IX, 1898, p. 257.
15

210

BOCK DISINTEGRATION AND DECOMPOSITION

feet below the surface and showed under the microscope a pre-
ponderance of bleached and iron-stained mica in scales so soft
and inelastic as to resemble talc. A few granules thought to be
pyroxene were still intact, while there remained an abundant
sprinkling of magnetite and perofskite. The serpentine of the
original rock had totally disappeared. The results of the an-
alyses and calculations are given below, the titanic oxide and
alumina, taken together, having been assumed as a basis for
comparison :

ANALYSES OP FRESH AND DECOMPOSED ALNOITE, HERKIMER COUNTY,

NEW YORK

III

VII

o S

CONSTITUENTS

Is?

W o

sip

H M

o w

goo

W ^ ^

^_, p o

^ Hj

fed
w

III

Ills

8 S

w^w

ell

SiO 2 . . . .

35.25

33.10

35.51

33.40

9.69

7-2.69

27.31

TiO 2 . . .
A1A . . .

2.25
6.10

2.90

7.88

2.27
6.14

2.93\
7.95J

0.00

100.00

0.00

Fe 2 3 . . .

FeO . . .

8.53
5.60

16.71

1.48

8.59
5.64

16.86 \
1.49 /

0.55

96.30

3.70

MgO . . .

20.40

13.42

20.55

13.54

10.08

51.03

48.97

CaO. . . .

7.40

5.25

7.46

5.30

3.36

54.90

45.10

K 2 0. . . .

2.88

0.29

2.90

0.29

2.68

7.73

92.27

Na 2 . . .

0.70

0,23

0.71

0.23

0.53

25.03

74.97

Ignition .

10.15

17.85

10.23

18.01

0.00

100.00

0.00

Total . .

99.26

99.11

100.00

26.89

The loss of 27.31% of the original silica in a rock so low in
alkalies is worthy of note, as is also the fact that the decompo-
sition, seemingly so thorough, was accompanied by a loss of but
26.89% of materials of all kinds. That 93.45% of the fresh rock
and 93.60% of the decomposed was soluble in dilute hydrochloric
acid and sodium carbonate solutions is of interest in connection
with what is stated on page 201. The maximum range of tem-
perature of the region is from about 6 to +95 Fahr., with
an annual precipitation of 43 inches.

WEATHERING OF PYROXENITES

211

Weathering of Soapstone. That pyroxenic rocks often un-
dergo alteration into talcose aggregates to which the name soap-
stone is applied, has been noted on p. 95. Such rocks on weath-
ering give rise to smooth-feeling, almost soap-like residues, which
like those derived from the peridotites are almost completely lack-
ing in lime and other alkalies. Two examples are given below :

ANALYSES OF FRESH AND DECOMPOSED SOAPSTONE, ALBEMARLE COUNTY,

VIRGINIA

III

CONSTITUENTS

RESIDUAL SOIL

PERCENTAGE OF
Loss FOB ENTIRE
EOCK

PERCENTAGB OF
EACH CONSTIT-
UENT SAVED

PERCENTAGE OF
EACH CONSTIT-
UENT LOST

Silica (Si0 2 ) ....

38 85 %

38 82 %

16 92 /

56 42 /

43 58 /

Alumina (Al 2 Os) . .
Iron sesquioxide (FegOs) 1
Lime (CaO)

12.77
12.86
6 12

22.61
13.33
6 13

0.00
6.33
2 66

100.00
58.52
55 55

0.00
41.48
44 45

Magnesia (MgO) . . .
Potash (K 2 O) ....
Soda (Na 2 0)
Ignition

22.58
0.19
0.11
6 52

9.52
0.18
0.20
9 21

17.20
9.03
0.00
1 32

23.81
52.94
100.00

7Q 74

76.19
47.05
0.00
20 9fi

100.00 %

52.46%

....

The fresh rock is of a blue-gray color, close texture, and
consists, as shown by the microscope, of elongated crystals of
colorless tremolite, with folia of talc and chlorite, and occasional
opaque granules of chromic iron. The general petrologic feat-
ures are those of an altered pyroxenite. The residual soil is of
a dull, ochreous, brown-red color, somewhat lumpy, but with
no appreciable grit when rubbed between the thumb and fingers.

Recalculated as before, the analyses give the results shown in
columns III, IV, and V.

The total loss of material amounts to 52.46%, including water
of hydration. The most striking feature brought out is the fact
that the magnesia has been carried away in greater proportional
quantity than has the lime. A like result was noted by Ebelman

All iron calculated as Fe 2 O 3 .

212

KOCK DISINTEGRATION AND DECOMPOSITION

in his analyses of the decomposed basalts of the Haute Loire,
which are given on p. 206.

A varietal form of this rock occurring near Fostoria in Fair-
fax County, the same state, is thoroughly decomposed throughout
nearly the entire area to a depth of twenty or more feet. The
fresh rock is composed mainly of a light greenish, almost white
talc, with sporadic patches of chlorite some five or more milli-
metres in diameter, and scattering granules of iron ores. The
decomposed material is dull brownish or gray, and when washed
and submitted to microscopic examination is found to consist
almost wholly of brown and yellow-brown scales of talcose
material, intermingled with an impalpable silt, composed so far
as determinable of talcose and chloritic shreds. It is wholly
without grit, and with a decided soapy or greasy feeling.
Analyses of fresh and decomposed material, and calculations as
already given, yielded results as shown in the accompanying
table.

ANALYSES OF FRESH AND DECOMPOSED SOAPSTONE, FAIRFAX COUNTY,

VIRGINIA

III

CONSTITUENTS

RESIDUAL SOIL

PERCENTAGE OP
Loss FOE ENTIRE
ROCK

PERCENTAGE OF
EACH CONSTIT-
UENT SAVED

PERCENTAGE OF
EACH CONSTIT-
UENT LOST

Silica (SiO 2 )

58.40%

64.84%

46 31 %

20. 70 %

79.30%

Alumina (A1 2 O 3 ) . . .
Iron oxides(FeO and Fe 2 3 )
Lime (CaO)

} 7.44
00

33.75
00

0.00

100.00

0.00

Magnesia (MgO) . . .
Alkalies (K 2 O and Na 2 O)
Ignition (H 2 O) ....

29.19
0.00
4.97

4.36
0.00
7.05

28.23
3.4i

3.29
31.28

96.71
68.72

100.00%

77.95%

THE WEATHERING OF SEDIMENTARY EOCKS

The principles involved in the decomposition of fragmen-
tal and crystalline stratified rocks are not so different from
those we have been discussing as to call for detailed considera-

WEATHEEING OF AEGILLITE 213

tion. It is well to note, however, that the materials composing
rocks of this type are themselves a product of these very dis-
integrating and decomposing agencies, but which have become
reconsolidated into rock masses and now, once more in the
infinite cycle of change, are undergoing a breaking up. It
follows from the very nature of the case that such rocks, with the
exception of the purely calcareous varieties, will undergo less
chemical change than do those we have been discussing. Their
feldspathic and easily decomposable silicate constituents long
ago yielded to the decomposing processes, and were largely re-
moved before consolidation took place. Thus, most sandstones
are composed largely of quartzose sand, the least soluble and
least changeable product, it may be, of many a previous disinte-
gration. Hence, the processes involved in the degeneration of the
sandstones, shales, and argillites, with the exception of those
which carry a feldspathic of calcareous cement, are largely me-
chanical. In these last-named, the cementing material gradually
gives way, and the rock becomes susceptible to the action of frost,
or falls away to loose sand simply through loss of cohesion.
Heusser and Claraz 1 described the itacolumites of Brazil as sub-
ject to this mechanical degeneration, the process being charac-
terized by fissuration, succeeded by complete disintegration.
Among siliceous sandstone it is the binding constituent that
yields first, as is naturally to be expected, and as has been
shown by the investigations conducted by R. Schutze. 2

Weathering of Argillite. The rocks grouped under the name
of argillites, though composed of detrital materials from pre-
existing rocks, and of particles reduced to an extreme degree of
fineness, are, nevertheless, quite variable in composition. As a
rule, they are among the most indestructible of rocks, and on
breaking down yield only clays which differ from the original
argillites mainly in degree of hydration and condition of oxida-
tion of the iron and other metallic constituents. Those argillites
which carry appreciable quantities of still undecomposed sili-
cates, particularly alkali-bearing varieties, are, of course, more
susceptible, other things being equal.

The deep blue-black argillites of Harford County, Mary-
land, as shown in the analyses given below, contain very con-

a Ann. des Mines, 5th, Vol. XVII, 1860.

2 Ueber Verwitterungsvorgange bei Krystallinisehen u. Sedimentargestei-
nen, Inaug. Dissertation der Friedrieh-Alexanders Universitat, Berlin, 1886.

214

EOCK DISINTEGEATION AND DECOMPOSITION

siderable quantities of undecomposed silicates, and though ex-
tremely tough and enduring from a human standpoint, in time
decompose in a very interesting manner. In the field these
rocks are found with their evident cleavage nearly vertical, and
forming steep, high ridges flanked by valleys carved from the
softer rocks on either hand. In the fresh cuts made during the
work of stripping, to open new quarries, the sound rock is found
overlaid by a variable thickness of ferruginous residual clay.
Joint blocks and splinters of the slate scattered through this
clay, in all stages of decomposition leave no doubt as to its
origin. Blocks, deep velvety black on the interior, are sur-
rounded by a crust of ochreous brown-red decomposition product,
the decay penetrating irregularly like the processes of oxidation
into a piece of metal. The first physical indication of decay is
shown by a softening of the slate, so that it may be readily scratched
by the thumb nail, and an assumption of a soapy or greasy feel-
ing, the entire mass finally passing over to the deep red-brown
unctuous clay, sufficiently rich in iron to serve as a low-grade
ochre, for paints. The incidental chemical changes are surpris-
ingly large, as shown by the analyses given below, column I

ANALYSES OP FRESH AND DECOMPOSED ARGILLITE, HARFORD COUNTY,

MARYLAND

III

CON8TITPBNT8

FRESH AHGILLITE

RESIDUAL CLAY

PERCENTAGE OF
Loss FOR EN-
TIRE KOCK

PERCENTAGE OF
EACH CONSTIT-
UENT BATED

PERCENTAGE OF
EACH CONSTIT-
UENT LOST

Silica 1 (Si0 2 )

44.15%

24.17%

25.34 %

42.43%

67.57%

Alumina (Al 2 0a)

30.84

39.90

0.00

100 00

Iron oxide (FeO and Fe 2 3 ) . .
Lime (CaO)
Magnesia (MgO)

14.87
0.48
0.27

17.61
None
0.25

1.23
0.48
0.08

91.22
0.00
71.84

8.78
100.00
28.16

Potash (K 2 0)

4.36

1 24

3 39

22 04

77 95

Soda (Na 2 O) .

0.51

0.23

99 64

Ignition (C and H 2 0) .

4.49

16.62

0.00

287.37

None

99.97%

100.02 %

40.83%

....

1 With traces of TiO 2 .
sulphur; hence no pyrite.

Manganese in traces, but not determined. No

WEATHERING OF CHERTS 215

being an average of two analyses of the black, little altered
material from the interior of one of these blocks, and II that of
the residual clay. In III, IV, and V are given the calculated
losses of constituents, as before. 1

This residual clay, when boiled with hydrochloric acid and
sodium carbonate solutions, yielded up nearly 70% of its matter
to these solvents, leaving a residue which, when examined under
the microscope, shows only faint yellow-brown scale-like par-
ticles, rarely over a tenth of a millimetre in diameter, acting
very faintly, if at all, on polarized light, and with borders often
serrate, through corrosion, though this latter feature may be
clue, in part, to the action of the solvents used. The niean an-
nual temperature of the region is 52.3 Fahr., and the rainfall
some 47.9 inches.

Weathering of Chert. Among siliceous sedimentary rocks
poor in alkalies or iron-bearing silicates the degeneration is
mainly disintegration, though a small amount of silica, existing
in either crystalline or chalcedonic forms, is usually lost through
solution. Thus the cherts of southwest Missouri break down into
porous friable forms, sometimes passing into the condition of
loose powder, or again retaining sufficient tenacity to be utilized
for filter discs and tubes, as at Seneca, in Newton County.

Analyses of fresh and altered forms of this material, as given
by Dr. E. 0. Hovey, 2 show no differences that are of sufficient
importance to warrant us in assuming any of them as the direct
cause of disintegration. The change is evidently mainly physical,
though it is more than probable that a certain amount of
interstitial silica has been removed. It is, of course, possible
that here, as in other forms of decomposition, extensive solution
may have taken place, leaving a residue which, so far as compo-
sition is concerned, gives no clew to the changes which have
occurred. Dr. Penrose, however, describes 3 a process of chert
decay, or more properly disintegration, as manifested in the
Batesville region of Arkansas, in which the cause of the break-
ing down is more apparent. There are two stages in the process,
as described: (1) A transition into a light, porous, opaque,
buff-colored rock of the consistency of ordinary pressed brick,
and (2) into an impalpable white or brown powder, locally

1 An analysis of a perfectly fresh slate from this locality is given on p. 119.

2 Appendix A, Vol. VII, Missouri Geological Survey, 1894, pp. 727-739.

3 Ann. Rep. Geol. Survey of Arkansas, Vol. I, 1890.

216 BOCK DISINTEGRATION AND DECOMPOSITION

known as a polishing powder. This second stage is not so con-
spicuous a feature as the first, since the finer materials thus
formed are carried off by surface waters. The white residual
powder often contains masses of the porous, semi-decomposed
rock, the latter in turn encircling kernels of hard, unaltered
chert. Throughout this region, the cherts (of Carboniferous
age) are generally decomposed into the condition of a more
or less porous mass to all depths up to ten or more feet.
In all cases the disintegration may be traced to the removal,
by leaching, of a small amount of interstitial carbonate of lime.

Weathering of Calcareous Rocks. When we come to a con-
sideration of the Calcareous rocks, we find, almost invariably,
the chemical agencies of degeneration preponderating over those
that are purely physical. In arid regions, and with granular
crystalline types, physical agencies may for a time prevail, but
as a rule the process is largely chemical, and notable for its
simplicity. The decomposition is due mainly to the action of
meteoric waters trickling over the surface, or filtering through
cracks and crevices, under ordinary conditions of atmospheric
pressure and atmospheric temperature. Hence the process is one
of superficial solution, and the incidental chemical processes set
in motion, as in the feldspar-bearing rocks, are almost entirely
lacking. It follows that only the lime carbonate is removed
in appreciable quantities, while the less soluble impurities are
left to accumulate in the form of ferruginous clays, admixed
with quartzose particles, chert nodules, etc. Since in many
limestones the amount of these constituents is reduced to a
minimum, even perhaps to the fraction of one per cent, so it
happens that hundreds, or even thousands of feet of strata may
disappear without leaving more than a very thin coating of soil
in their place.

An interesting illustration of the changes taking place in the
decomposition of an impure Carboniferous limestone is described
by Penrose in his treatise on the genesis of manganese deposits. 1
The stone in its least changed condition is of a granular crys-
talline structure and dark chocolate-brown color. The residual
clay from its decomposition is a trifle darker, highly plastic, and
quite impervious. On the next page are given the analyses of (I)
the fresh rock and (II) the clay, both being taken from the same
pit, the latter being of about fifteen feet in thickness and over-

x Ann. Eep. Geol. Survey of Arkansas, 1890, p. 179.

WEATHERING OF CALCAREOUS ROCKS

217

laid by a capping of chert, which reduced to a minimum the
possibility of any admixture of foreign matter. The materials
were dried at a temperature of 110 to 115 C. before analyzing.

ANALYSES OF FRESH LIMESTONE AND ITS RESIDUAL CLAY, BATESVILLE

ARKANSAS

III

CONSTITUENTS

FRESH
LIMESTONE

EESIDUAL CLAY

PERCENTAGE OF
Loss FOR ENTIRE
KOCK

PERCENTAGE OF
EACH CONSTIT-
UENT SAVED

PERCENTAGE OF
EACH CONSTIT-
UENT LOST

Silica (SiO a )
Alumina (Al 2 Os) . .
Ferric iron (Fe 2 3 ) . .
Manganic oxide (MnO) .
Lime (CaO)

4.13%
4.19
2.35
4.33
44.79

33.69 %
30.30
1.99
14.98
3.91

0.00^
0.47
2.11
2.49
44.32

100.00 %
88.65
10.44
42.41
1.07

0.00%
11.35
89.56
67.59
98.93

Magnesia (MgO) . . .
Potash (K 2 0) ....
Soda (Na 2 O) . ...

0.30
0.35
0.16

0.26
0.96
0.61

0.27
0.23
0085

10.62
33.63
46.74

89.38
66.37
53 26

Water (H 2 0)

2.26

10.76

0.95

58.37

41.63

Carbonic acid (C0 2 ) . .
Phosphoric acid (P 2 O 5 ) .

34.10
3.04

0.00
2.54

34.10
2.73

0.00
10.24

100.00
89.76

100.00 %

87. 755 &

These analyses have been recalculated in the same manner as
before, excepting that silica, instead of alumina, is taken as the
constant factor. It is believed that one is safe in assuming little
or no silica is lost here through the action of alkaline carbonates,
since the alkalies are almost wholly lacking in the fresh rock,
and a large portion of the silica doubtless exists as free quartz.
Recalculating, then, in the same manner as before, but on a silica
constant basis, we obtain the matter in columns III, IV, and V.

These columns bring to light some unexpected features, not
the least interesting of which is the fact that the residual clay,
in spite of its highly hydrated condition, in reality contains
scarcely half the amount of water it would, had the small amount
(2.26%) in the original limestone been allowed to accumulate
without loss. A more important, though perhaps more to be ex-
pected, feature is the entire removal of that portion of the lime
which existed as carbonate, as indicated by the absence of car-

218 BOCK DISINTEGRATION AND DECOMPOSITION

bonic acid in the clay. It will be noted that 87.75% of the
entire rock mass has disappeared through leaching, leaving only
12.24% to accumulate as an insoluble residue in the form of soil.

A compact, deep blue-gray limestone belonging to the Trenton
period and occurring near Hagerstown, Maryland, leaves a deep
red, clayey soil, poor in lime and containing but the less soluble
constituents of the parent rock. Subjected to analyses and
calculations as above, this showed a total loss of materials
amounting to 98.75%. Another sample (a magnesian limestone
from near Staunton, Virginia) suffered a loss of but 90.76%, and
showed some minor differences which it may be well to note in
detail. The residual clay is of a deep-red color, highly plastic,
and on drying becomes so hard as to be broken only by means of
a hammer.

So abundant is the iron oxide that the mineralogical nature of
the residual material is stained almost beyond recognition, and
it is only when it is first boiled in dilute hydrochloric acid to
remove the iron, that it can be studied at all satisfactorily.
When thus treated and submitted to microscopic examination,
it is found to consist mainly of very irregularly rounded and
angular quartz fragments, which are more or less corroded and
unmistakably of clastic origin, i. e., they existed in the limestone,
not in the form of particles crystallized in place, but as me-
chanically included detritus formed from the breaking down of
pre-existing siliceous rocks. Particles of feldspar, a portion of
which show twin banding, are also present and, more rarely, are
found shreds of white and black mica, chlorite, epidote, and,
very rarely, a minute but very perfectly preserved, doubly ter-
minated colorless crystal, with forms characteristic of rutile.
Both quartzes and feldspars are roughened and corroded, though
even the plagioclase feldspars are still in many cases sufficiently
fresh to show twin striae. The analyses and calculations based
thereon are given on the next page. 1

It is to be noted that in this case the magnesia and lime were
lost in nearly equal proportions. The fact that so large a pro-
portion of the alkalies remain in the clay is due to their being
constituents of the silicate particles which did not decompose as
rapidly as the lime and magnesia were removed in solution.

1 Analyses recalculated on basis of 100 and all the iron considered as
ferric. With the silica is included 0.09% TiO 2 . See Bulletin 150, U. S.
Geological Survey.

WEATHERING OF CALCAREOUS ROCKS 219

ANALYSIS OF FRESH LIMESTONE AND RESIDUAL CLAY, STAUNTON, VIRGINIA

o W 1 ^

IgH

PH ft*

K H

P5 O H

&<&

(>Hfc

KM a

SiO 2 + TiO 2 l

7.41 %

57.57^

2.03^

72.61^

27.39^

A1 2 3 ....

1.91

20.44

o.oo

100.00

0.00

Fe 2 3 (2)

0.98

7.93

0.29

75.11

24.89

CaO

28.29

0.51

28.24

0.17

99.83

MgO

18.17

1.21

18.06

0.62

99.38

K,0

1.08

4.91

0.62

42.51

57.49

Na 2

0.09

0.23

0.07

23.96

76.04

Co,

41.57

0.38

41.53

0.85

99.15

0.03

0.10

0.02

31.22

69.78

H 2 O .

0.57

6.69

0.55 1

100.00

90.86

This leaching out of the lime carbonates and the accumula-
tion of insoluble residues is a strikingly conspicuous feature in
regions abounding in limestone caverns, and to it is due the
tenacious ferruginous clays which cover their floors. So rich
indeed are some of these residual deposits in iron oxide that
in some instances they are locally used for pigments, under the
name of ochre or mineral paint, or again, where occurring in
large quantities, as ores of iron. (See p. 100.)

% It is possible that loosely consolidated beds of shell limestone
may undergo a process of change, perhaps more nearly akin to
alteration than decomposition, through agencies quite different
from those we have been considering. Darwin found the shells
in the raised sea-beaches of San Lorenzo, South America, altered
to the condition of a white powder without trace of organic
structure, and consisting of carbonate, sulphate and chloride
of lime with sulphate and chloride of sodium. This alteration he
believed to be due to a mutual reaction taking place between the
original sodium chloride derived from the sea-water and the
lime carbonate of the shells, and he speaks of it as an interesting
illustration of the fact that the dry climate of the west coast of

1 Gain.

220 EOCK DISINTEGRATION AND DECOMPOSITION

South America is much less favorable to the preservation of
shell structures than would be a moist one where the salt would
be removed too rapidly for the double decomposition to be
brought about.

Resume. Making all due allowance for possible sources of
error in our methods, there are certain general deductions that
may be safely drawn. Not, it may be, from our own analyses
alone, but from numerous others as found in existing literature. 1

Let us briefly review the subject and make the deductions
accordingly.

In glancing over the analyses, it is at once apparent that
hydration is an important factor, the amount of water increas-
ing rapidly as decomposition advances. In the earlier stages of
degeneration it is doubtless the most important factor. 2 There
is, moreover, among the siliceous crystalline rocks, in every case
a loss in silica, a greater proportional loss in lime, magnesia, and
the alkalies, and a proportional increase in the amounts of
alumina and sometimes of iron oxides, though the apparent
gain may in some cases be due to the change in condition from
ferrous to ferric oxide. As a whole, however, there is a very
decided loss of materials. Among siliceous crystalline rocks,
this loss, so far as shown by available analyses and calculations,
rarely amounts to more than 60% of the entire rock mass.
Among calcareous rocks, on the other hand, it may, in extreme
cases, amount to even 99%.

Of all the ordinary essential mineral constituents the free
quartz is the most refractory toward purely chemical agen-
cies, and the amount of silica lost from this source must be
small, though Sorby 3 thinks to have distinguished chemically
corroded quartz granules in some of the sands examined by him.
It is, however, safe to say that the mineral suffers chiefly from
mechanical disruption, that silica in any rock which is re-
moved during the process of decomposition is derived mainly
from the silicates, and not from the free quartz. According to
Bischof, and as shown by our own work, the silicates that are

1 See especially Both 's Allegemeine u. Chemische Geologic, Vol. Ill, and
Ebelmen's papers in Ann. des Mines, Vols. VII, 1845, and XII, 1847.

2 Hydration stands as the most extensive reaction in the belt of weathering.
In its importance in this belt, as a geological process, it is second only to
carbonation. (Van Hise, Treatise on Metamorphism, p. 481.)

8 Proc. Geol. Soc. of London, 1879.

GENERAL DEDUCTIONS 221

most readily decomposed are those containing protoxides of iron
and manganese, or lime, and the first indication of decomposi-
tion is signalled by a ferruginous discoloration and the appear-
ance of calcite. The evidence bearing upon the relative dura-
bility of the various minerals constituting rocks is, however,
quite conflicting and unsatisfactory. Doubtless much depends
on local conditions.

Dana observed 1 that in the decomposition of the granitic
rocks of the Chilean coast the feldspars yielded first, becoming
white and opaque and of a friable earthy appearance. But it
should be noted that there is nothing in Professor Dana's de-
scription to show that this change may not have been a purely
physical one, and due to the splitting up of the feldspars along
cleavage lines. Fournet, from a study of the processes of kao-
linization, was led to state 2 that hornblende yields less readily
to decomposing forces than does feldspar, when the two are
associated in the same rock. Becker, however, in studying
deep-seated decomposition in the Comstock Lode of Nevada,
arrived at a precisely opposite conclusion, the feldspars as a
whole offering, more resistance than the augite, hornblende, or
mica. Lindgren noted 3 that the decomposition of the California
grano-diorite manifested two distinct phases. The first, due to
the decomposition of the feldspar grains alone resulted in re-
ducing the rock to a soft, crumbling mass. In the second stage,
the biotite and hornblende also are decomposed, the biotite being
the more refractory.

The present writer has described 4 thick sheets of augite por-
phyrite in Gallatin County, Montana, in which the feldspathic
disintegration has gone so far that the mass falls away to a
coarse sand, from which still perfectly outlined crystals of coal-
black augites may be gleaned in profusion. This last is,
however, a semi-arid region, and the process thus far one of
disintegration more than decomposition. In a moist, or perhaps
in any climate, minerals consisting essentially of silicates of
alumina and magnesia are less liable to decomposition than
those containing considerable proportions of iron protoxides or
of lime. This for the reason that the first-named are scarcely

Report Wilkes's Exploring Expedition, Geology, p. 578.

2 Ann. de Chimie et de Physique, Vol. LV, 1833, p. 240.

8 Seventeenth Ann. Eep. U. S. Geol. Survey, Part II, 1895-96, p. 39.

4 Bull. U. S. Geol. Survey, No. 110, 1894.

222 KOCK DISINTEGRATION AND DECOMPOSITION

at all affected by the ordinary atmospheric agents of solution.
Bischof goes so far as to say that the silicate of alumina is not
at all affected by carbonic acid, but the researches of Miiller, to
which reference has been made, and our own investigations, tend
to disprove this. Dana states 1 that in the decomposition of
basalt, on the island of Tahiti, the olivine is the earliest to give
way, becoming first iridescent and finally falling away to a soft,
pulverulent, ochreous yellow or brown powder. The compact
base of the rock yielded next, the augite holding out until the
last. Those silicates which are least liable to atmospheric de-
composition are, as is to be expected, those which have resulted
from the alteration of less stable silicates, as serpentine from
olivine, epidote from hornblende, or kaolin from feldspar, etc.
A few silicates like tourmaline and zircon, or garnet, or oxides
like rutile and magnetite, or the salts of rarer earths like mona-
zite and zircon, are scarcely at all affected by any of the ordi-
nary agents of decomposition, but remain in the form of residual
sands in the beds of streams, from whence the lighter, more
decomposed material is removed by erosion.

In the weathering of potash-feldspar rocks carrying black
mica, the latter mineral is as a rule the first to give way, and at
times almost wholly disappears. With basic rocks, on the other
hand, the dark mica is one of the most enduring of the constitu-
ents, and in the residual sands may be found in surprisingly
large proportions.

In the kaolinized gneisses of northern Delaware, the biotite,
as a rule, is in an advanced stage of decomposition, while the
small amount of primary muscovite is still fresh and intact,
retaining all its original lustre and elasticity.

Among the feldspars the potash varieties are far more re-
fractory than the soda-lime, or plagioclase varieties. This is
shown not merely by our own investigations, but by those of
others as well. Roth shows 2 from analyses of fresh and weathered
phonolites, nepheline basalts, and dolorites, that the loss of soda
is almost invariably greater than that of potash.

In the coarse, pegmatitic dikes of Delaware County, Penn-
sylvania, the microcline masses, as mined for pottery purposes,
are beautifully fresh and translucent, while the associated oligo-
clase is snow-white through a splitting up along cleavage lines

1 Op. cit., p. 298.

*0p. cit., 3d ed., 2d Heft.

GENERAL DEDUCTIONS 223

and partial decomposition. Where thrown out upon the dumps,
this whitened mineral shortly falls away to fine sand, resembling,
at first glance, kaolin, but is distinctly gritty.

Max Geldmacher noted 1 that in the weathering of quartz
porphyry the oligoclase always gave way before the orthoclase.

Indeed, as shown in our analyses, in certain phases of rock
degeneration, the potash feldspars may lose very little by
decomposition, but be converted into the condition of fine
silt merely through a mechanical splitting iip. This fact will
in part explain the relative scarcity of free potassium salts
(carbonates, sulphates, and nitrates) as compared with those of
soda.

The chemical processes involved in this feldspathic decompo-
sition are of sufficient importance to warrant further discussion,
even though it may involve a certain amount of repetition of
what has gone before.

Berthier, Forschammer, Brongniart, 2 Fournet, 3 and others ex-
plained more than fifty years ago the process of feldspathic dis-
integration through the breaking up of its complex molecule
into alkaline silicates soluble in water, and aluminous silicates
which are insoluble. The loss in silica, as noted above, was
supposed to be due to the removal, by solution, of these alka-
line silicates. Ebelman, 4 however, subsequently showed that sili-
cate minerals poor or quite lacking in alkalies lost a portion of
their silica, as is also shown in the analyses of altered pyroxenites
on pp. 211 and 212. He accounted for this on the supposition
that the silica set free in a nascent state was soluble either
in pure water, or water containing carbonic acid. This observa-
tion is corroborated by the work of Kahlenberg and Lincoln, 5
who showed m 1898 that in very dilute solutions such as natural
mineral solutions must necessarily be, the silica is present in a
colloidal form and not as silicic acid. Bischof states in his
earlier work (1856) that when meteoric waters containing car-
bonic acid filter through rocks containing alkaline silicates, the
first action consists in the partial decomposition of these sub-
stances by the carbonic acid and the formation of alkaline car-

'Beitrage zur Verwitterung cler Porphyre, Inaug. Dissertation, Konigl.
Freidrich Alexander Universitat, Leipzig, 1889.

2 Arch, du Museum, Vol. I, 1839 (cited by Ebelmen).

3 Ann. de Chimie et de Physique, Vol. LV, 1833.
* Ann. des Mines, Vol. VII, 1845.

6 As quoted by Cameron and Bell, op. cit., p. 19.

224 KOCK DISINTEGRATION AND DECOMPOSITION

bonates, which are dissolved. If the water thus impregnated,
on penetrating further below the surface, comes in contact with
calcareous silicates, another change will take place consisting of
a decomposition and replacement of these calcareous silicates by
the alkaline silicates, and a removal of the lime set free, as a
carbonate, provided the water still contains a sufficient amount
of carbonic acid. This replacing process and the retention of
the alkaline silicates is accounted for on the supposition that,
in their nascent state, they form new combinations with the
other silicates present, while the lime remains as a carbonate to
be removed or not, as the case may be. He further states that
the alkaline carbonates originating in the manner described
are among the most soluble substances known; the carbonate
of soda requires for solution only six times its weight of water
at ordinary temperatures. Silica, on the other hand, even in
its most soluble form, requires ten thousand times its weight of
water for solution. If, therefore, the decomposition of feld-
spar by such carbonated water were ever so energetic, there
would be sufficient water for the solution of the carbonate of
soda formed. But if the silica separated meanwhile amounted
to more than y^^ of the water present, the excess could not
be dissolved, but would remain mixed with the kaolin.

The case is very different when the decomposition of feldspar
is affected by fresh water containing only the minute quantity
of carbonic acid derived from the atmosphere. By the action
of such water, only very small quantities of alkaline carbonates
are formed; consequently it is possible that the silica separated
at the same time, also small in quantity, may find enough water
for solution. In such cases the whole of this silica would be
removed with the alkaline carbonates, and pure kaolin would
be left. Such an action as this does not, however, appear to
take place; for the purest of kaolin nearly always contains an
admixture of quartz sand, or of free silica in some of its forms.

K. V. Murakozy has shown 1 that in the decomposition of
rhyolite from Nagy-Mihaly, the sanidin passes into kaolin and
opal, the latter separating out as hyalite in veins or impure
concretionary forms.

It follows from this consideration that in the decomposition
of feldspar into kaolin more of the silica separated remains
mixed with the kaolin formed, the greater the quantity of

1 Abstract by F. Becke, Neues Jahrbuch, 1894, 1 Band, 2 Heft, p. 291.

GENERAL DEDUCTIONS 225

carbonic acid in the water, and that, perhaps, the amount of car-
bonic acid is never so small that the whole of the silica sep-
arated in the decomposition of feldspar can be removed. 1 The
above, however, overlooks the possible presence of nitrates noted
on p. 154. The larger the proportion of nitric acid the greater
would be the amount of silica intermingled with the kaolin,
since whatever proportion of the alkalies failed to be carried
away as nitrates would pretty certainly disappear as carbonate.
There is also the possibility, especially in the rocks rich in iron
protoxides, that a portion of the silica may combine with the
iron, as already noted.

In cases where the decomposition takes place under the
influence of a sufficient supply of oxygen, all iron, and presum-
ably the manganese as well, would be converted into the in-
soluble hydrous sesquioxide form and remain with the residue.
Where, however, the supply of oxygen is insufficient, a por-
tion or all of these constituents may be removed in the form
of protoxide carbonates, or, in the case of iron, of a ferrous
sulphate. These facts well account for the variation in sta-
bility of the iron, as indicated in the preceding analyses.

Reference has already been made to the fact that the mag-
nesia from the decomposition of magnesian silicates was some-
times removed in greater relative portions than was the lime.
This seeming anomaly is also sometimes met with in cal-
careous stratified rocks. Both 2 showed that in the weather-
ing of dolomitic limestones, the magnesia is often removed in
greater proportional quantities than the more soluble lime
carbonate.

The researches of Hitterman 3 showed, however, that carbonic
acid solutions may exert a scarcely appreciable effect upon mag-
nesian carbonate, which therefore accumulates in the residual
soils.

It is safe to say that while the general process of rock-
weathering may be quite simple, as outlined, there are many
minor reactions which it is not possible to describe in detail.

It has been shown that even in firm rocks a mutual chemical

1 Chemical and Physical Geology, by Gustav Bischof, Vol. II, pp. 382,
183.

2 Op. cit., Vol. III.

8 Die Verwitterungeproducte von Gesteinen der Triasf ormation Frankers,
Inaug. Dissertation, Freidrich-Alexanders Universitat, Munich, 1889.
16

226 EOCK DISINTEGRATION AND DECOMPOSITION

reaction is not uncommon among minerals lying in close juxta-
position, giving rise to what are known as reaction rims or zones
composed of secondary minerals. This is a particularly con-
spicuous feature in many gabbros, where olivine and feldspar
are closely adjacent. In these cases, a mutual interchange of
elements may take place, giving rise to garnets, free quartz, or
other minerals as the case may be. This is, to be sure, a deep-
seated change, to be classed as alteration rather than decomposi-
tion, and taking place presumably under conditions of tempera-
ture and solution quite at variance with those existing on the
immediate surface. It is, nevertheless, self-evident that when
elements are set free through any process, they must almost im-
mediately recombine, taking those forms which existing circum-
stances may dictate and that close contact of particles would be
favorable to the more rapid formation of new compounds. In
a mass of decomposing rock, circumstances are almost continu-
ally changing, and the inference is fair that new combinations
are continually being made and unmade, the intricacies of which
we are unable to follow.

PLATE 18

FIG. 1. Exfoliated granite in the Sierras.

FIG. 2. Eock basin in granite formed by weathering.

FIG. 3. Disintegrated granite, Ute Pass, Colorado.

THE WEATHERING OP ROCKS (Continued)

III. THE PHYSICAL MANIFESTATIONS

Rock-weathering manifests itself in a great variety of ways,
much depending upon climate, though naturally the controlling
factor is that of mineral composition. The manner of weather-
ing is often sufficiently characteristic to be of great importance
in determining surface contours, as well as incidentally afford-
ing a means for the identification of rock masses when the
outcrops themselves are obscured by decomposition products
Such a means is of only local importance, however, since under
varying conditions the resultant forms assumed, even by similar
rocks, are themselves quite variable. It is, nevertheless, not
without interest to note the varying phases of weathering in
different kinds of rocks, the incidental contours assumed, the
character of the resultant debris, and, at the same time, the
controlling forces that have been instrumental in bringing about
the final result.

(1) Disintegration without Decomposition. That in weath-
ering, physical and chemical agencies may go on either singly
or conjointly has been noted in previous pages. In the case of
single minerals, the preliminary disintegration is beautifully
illustrated in the large oligoclase masses associated with micro-
cline in the feldspar nifties of Delaware County, Pennsylvania.
In the dumps of waste about the mines these are found, in all
stages of disintegration, the mineral splitting up along cleavage
lines, becoming slow-white, and ultimately falling away to a
kaolin-like product, but which, when submitted to microscopic
examination, is found to be made up of sharply angular cleavage
particles, showing little sign of decomposition other than that
indicated by occasional opacity. In the analyses given on the
next page are shown (I) the composition of a fresh oligoclase
(as given by Dana) from near Wilmington, Delaware, (II) the
snow-white cleaved, but still moderately firm mineral mentioned
above, and (III) the flour-like or kaolin-like product.

227

228 THE PHYSICAL MANIFESTATIONS OF WEATHERING

ANALYSES OF FRESH AND DISINTEGRATED OLIGOCLASE, WILMINGTON,

DELAWARE

III

CONSTITUENTS

FRESH OLIGOCLASE

OPAQUE WHITE,
BUT STILL FIRM
OLIGOCLASE

FINE DUST FROM
DISINTEGRATED
OLIGOCLASB

Si0 2

64.75%

61.23%

66.73%

AlgOs . . .

23.56

25.65

28.44

CaO

2.84

2.37

2.95

K 2 O.

1.11

0.72

1.12

Na 2 O

9.04

7.66

5.81

Ignition

1.00

5.67

101.30 %

98.63%

100.72%

The fact that granitic and gneissic rocks may undergo ex-
tensive disintegration with slight decomposition, even in a
moist climate, was noted by Nordenskiold 1 in Ceylon. He
says: "The boundary between the unweathered granite and
that which has been converted into sand k often so sharp that
a stroke of the hammer separates the crust of granitic sand
from the granite blocks. They have an almost fresh surface,
and a couple of millimetres within the boundary the rock is quite
unaltered. No formation of clay takes place and the alteration
to which the rocks are subjected, therefore, consists in a crum-
bling or formation of sand, and not, or at least only to a very
small extent, in a chemical change. At every road section
between Galle, Colombo, and Eatnapoora the granite and gneiss
crumbled down to a coarse sand, which was again bound to-
gether by newly formed hydrated peroxide of iron to a peculiar
porous sandstone, called by the natives cabook. 2 This sandstone
forms the layer lying next the rock in nearly all the hills on that
part of the island which we visited. It evidently belongs to
an earlier geological period than the Quaternary, for it is older
than the recent formation of valleys and rivers. The cabook
often contains large, rounded, unweathered granite blocks, quite
resembling the rolled stone blocks in Sweden. In this way
there arises at places where the cabook stratum has again

1 Voyage of the Vega, Vol. II, 1881, p. 420.

2 Laterite? It seems so regarded by H. F. Alexander, Trans. Edinburgh
Geol. Society, Vol. II, 1869-74, p. 113.

WEATHERING INFLUENCED BY STRUCTURE 229

been broken up and washed away by currents of water, forma-
tions which are so bewildering, like the ridges (osars) and hills
with erratic blocks in Sweden and Finland, that I was aston-
ished when I saw them. ' '

The same features are brought out in the previous descrip-
tions relative to the weathering of the granite of the District
of Columbia, the diabase of Medford, Massachusetts, and other
.localities mentioned in these pages. (See pp. 186 and 198.)
This tendency toward disintegration without decomposition is
exaggerated among coarsely crystalline rocks, as is abundantly
exemplified in the rocks of the Pikes Peak (Colorado) area.
Among those of finer grain, particularly the quartz-free varieties,
as the Fourche Mountain (Arkansas) syenites, decomposition
may follow so closely on disintegration that little or no sand
is formed, sound fresh rock passing within the space of a few
millimetres into the condition of residual clay. 1

(2) Weathering influenced by Crystalline Structure. it is
elsewhere observed that, other things being equal, a coarsely
granular rock will disintegrate more rapidly than one of finer
grain.

Lone Mountain, one of the high eruptive peaks on the west
side of the Madison valley in Montana, presents in its upper
portions all the features of a volcanic crater broken down on
one side by the lava flow. The facts of the case are, however,
that the coarser grained central portion has been disintegrated,
and swept by wind and rain into the valleys, while the fine-
grained, more compact outer portions, those which solidified near
the line of contact with adjacent rocks, remain intact. Pro-
fessor Bell 2 describes an interesting case of this kind where the
coarsely crystalline central portion of a "greenstone" dike has
yielded more readily to erosion than at the sides and afforded
channel-way for the Mattagami River, north of Lake Huron, in
Canada. The gneiss adjoining the dike having been shattered,
yielded also to decomposing agencies and forms now a second
parallel channel on each side of the central one. ' ' Between them

1 Dr. Max Fesca has noted that the granitic rocks of Kai province, Japan,
yield on decomposing gravel, sand, and clayey loams, while those rocks poor
in quartz, such as the syenites, give rise only to clays (Abhandlungen und
Erlauterungen zur Agronomischen Karte de a Prov. Kai, Kaiserlich Japan-
ischen Geologischen Reichsanstalt, 1887).

'Bull. Geol. Soc. of America, Vol. V, 1894, p. 364.

230 THE PHYSICAL MANIFESTATIONS OF WEATHEEING

the finer grained, hard, and undecayed 'greenstone' constituting
the outer portions of the dike rises up in the shape of ridges
and chains of islands, so that the riyer flows as a main, central
channel, more or less separated from the smaller lateral ones."
The same writer describes several instances in which long straight
valleys in the Archaean regions of Canada, now occupied by
straight river stretches, long narrow lakes or inlets of the larger
lakes, are due to the decay and removal of the wide "greenstone"
dikes, or of parallel dikes with narrow belts of rock between.
Long Lake, north of Lake Superior, some 52 miles in length, is
mentioned as typical of lakes of this class.

(3) Weathering influenced by Structure of Rock Masses. -
In any rock mass weathering is greatly augmented by lines of
weakness, such as joint and bedding planes, since these furnish
so many additional points of attack. In homogeneous massive
rocks the rate of disintegration is retarded by a lack of vulner-
able points, and the resultant form is that of rounded bosses
such as are shown in plate 1.

As a rule, however, the most massive of rocks are traversed
by one or more series of joints (see Pis. 16 and 19) whereby they

FIG. 17. Showing the influence of joints in the production of boulders.

are divided up into rhomboidal blocks of varying sizes. Even
when not sufficiently developed to be conspicuous, such joints
may exist as lines of weakness along which moisture and the ac-
companying agents of disintegration make their way, gradually
rounding the corners until there are left but the oval boulder-
form masses of which the so-called ' ' niggerheads " of the gabbro
area about Baltimore are typical examples. In nearly all such
rocks the exfoliation and decomposition take place in the form
of concentric layers, like the coatings on an onion. This holds
true with the huge granitic bosses, as well as with the smaller

-a

UNIVERSITY

WEATHERING INFLUENCED BY STRUCTURE

231

joint blocks, and has been argued by some of the earlier geologists
as indicative of an original concretionary structure. Such an
assumption seems, however, wholly uncalled for. If the block or
mass is reasonably homogeneous, the agencies of decomposition
will penetrate nearly uniformly from all equally exposed sur-
faces, producing an exfoliation nearly parallel to that surface,
and the concentric structure is inevitable, as was long ago
pointed out.

In some cases the tendency to assume the boss-like form is
accentuated through the presence of joints running approxi-
mately parallel to the exposed surface, such joints as give rise
to the step-like arrangement of the stone so frequently seen in
granite quarries. Stone Mountain, Georgia, an immense boss
of* light gray granite some 2 miles long by 1J wide and 650 feet

FIG. 18. Exfoliation of granite.

high, owes its form, apparently, wholly to exfoliation parallel
to pre-existing lines of weakness. The entire mass, so far as
exposed by quarrying operations, is made up of imbricated sheets
of granite, which, of unknown thickness beneath the surface,
thin out to mere knife edges above, like shingles on a roof.
Through prolonged exposure the superficial layers have become
detached from the parent mass, and doubtless hundreds of feet
in vertical thickness completely disintegrated and swept away.
With many geologists these joints, in themselves, would be ac-
cepted as due to atmospheric action. The boss-like form is there-
fore incidental and consequent. The process of exfoliation has,
in the case mentioned, been productive of some peculiar results
which may be described in detail.

232 THE PHYSICAL MANIFESTATIONS OF WEATHEKING

As above mentioned, the sheets of granite, varying from a few
inches to several feet in thickness, conform in a general way to
the present surface of the hill. Constant expansion and con-
traction from temperature changes have, in the manner already
described, so expanded these sheets that, bound at the sides,
they have found relief in an upward direction where resistance
was least, and risen in dome or roof shaped forms, as shown in
the sketch. (Fig. 18.) The weight of the sheets higher up the
slopes, impinging upon the edges of those below, has in some
cases undoubtedly aided in the work, but the larger part is due
to simple expansion, such as was referred to on p. 159.

These ruptured sheets are rarely more than 10 inches thick,
but 10 or 20 feet in diameter. The material, though quite fresh
appearing, is loosely granular and friable, easily reduced to sand.
This same mass of granite sometimes shows upon its surface
peculiar circular depressions, one within another, separated by
intervening ridges of low relief, such as have been described in

a much more perfect
stage of development by
Dr. Robert Bell 1 in the
Huronian rocks of Can-
ada. These, as shown in
Fig. 19 from Bell's paper,
are some 3 or 4 feet in
diameter and 3 or 4 inches
high. The cause of this
form of weathering at
Stone Mountain is not ap-
parent, though Bell, in the
case figured, regards it as
induced by an original
concretionary structure.

The spheroidal struc-
FIG. 19. ture so frequently seen

in basaltic rocks, and

as typified in the sphaeroidische absonderung of German writers,
may perhaps be due to an original spheroidal tendency caused
by cooling, 2 but a very large proportion of the spheroidal masses

'Bull. Geol. Soc. of America, Vol. V, 1894, p. 362.
2 T. G. Bonney, Quar. Jour. Geol. Soc. of London, Vol. XXXII, 1876, p.
153.

WEATHERING INFLUENCED BY STRUCTURE 233

so typical of the decomposition of massive rock is, as already
suggested, due wholly to external causes. W. P. Blake in 1855
called attention to this form of disintegration in the massive
sandstones near San Francisco (California) and pointed out
the true explanation. 1

This sandstone is described as occurring in the form of layers
from a few inches to 6 and 8 feet in thickness, alternating with
beds of slate and shale. Down to a depth of 10 or 20 feet, or
to the limits of atmospheric action, all the beds have turned from
gray to rusty brown or drab. i ' There are, however, parts of the
upper beds that have not yet been reached and changed by de-
composition ; these parts are found in the condition of spherical
or ellipsoidal masses, from which the weathered parts scale off
in successive crusts. These nuclei have the appearance of great
rounded boulders, and have accumulated in great numbers at
the base of the cliff/' In this case the sandstone is composed
mainly of grains of quartz and a little feldspar cemented by
calcite, the disintegration being due mainly to the removal of
this cement by percolating water, while the change in color is
doubtless due to oxidizing pyrite or ferrous carbonate.

The effect of percolating waters is not, however, always im-
mediately destructive. Through the presence of cementing ma-
terials in solution or by causing an oxidation of the iron car-
bonates or sulphides, a local induration may be induced along the
joint lines such as becomes conspicuous only through the weath-
ering away of the non-indurated portions. Resultant forms may
be extremely regular or again irregular, according to the char-
acter of the lines along which percolation takes place, and that
of the rock itself. An interesting illustration of this form of
weathering is that given by Wyville Thompson 2 as occurring
in limestones on the islands of Bermuda.

"This dissolving and hardening process," he writes, "takes
place irregularly, the water apparently following certain courses
in its percolations, which it keeps open, and the walls of which
it hardens; and in consequence of this, the rock weathers most
unequally, leaving extraordinary rugged fissures and pinnacles,
and piled up boulders, the cores of masses which have been

1 Expl. and Survey for a Railroad from the Mississippi to the Pacific
Ocean; Report on the Geology of the Route, near the 32d Parallel, by W.
P. Blake.

2 See The Atlantic, Vol. I. Also Bull. 25, U. S. National Museum.

234 THE PHYSICAL MANIFESTATIONS OF WEATHEEING

eaten away, more like slags or cinders than blocks of limestone.
The ridges between Harrington Sound and Castle Harbor are a
good example of this. It is like a rockery of the most irregular
and fantastic style, and there seems to be something specially
productive in the soil ; for every crack and crevice is filled with
the most luxuriant vegetation, mossing over the stones and train-
ing up as tier upon tier of climbers, clinging to the trees and
rocks. Frequently the percolation of hardening matter, from
some cause or other, only affects certain parts of a mass of rock,
leaving spaces occupied by free sand. There seems to be little
doubt that it is by the clearing out of the sand from such
spaces, either by the action of running fresh water or by that
of the sea, that those remarkable caves are formed which add
so much to the interest of the Bermudas."

A form of weathering due to similar causes, but productive
of results much more regular in arrangement, is shown in
Fig. 4, PL 22, from a block of weathered sandstone in the
National Museum. The original joints through which the
waters filtered are easily recognized in the sharp straight lines
running diagonally across the specimen. Blocks of fine shale
and argillite, in their incipient stages of weathering, often show
concentric bands of varying color, due to the oxidizing effect
of water percolating inward from all sides of the natural joints
as shown in Fig. 3, PI. 22.

In stratified rocks there is, as a rule, a lack of homogeneity,
certain layers being more porous than others, or containing
mineral constituents more susceptible to the attacking forces.
Such rocks, therefore, weather unevenly, and give rise to ex-
ceedingly ragged contours. The finely fissile schists standing
nearly on edge along the coast of Casco Bay, in Maine, under
the combined influence of wave and atmospheric action, weather
into peculiarly fantastic forms resembling nothing more than
piles of old lumber in which the multitudinous channels formed
by boring coleopterous larvae have become irregularly enlarged
by decay. (See Fig. 1, PL 20.) The numerous quartz veins by
which these schists are traversed stand out in bold relief until
no longer supported by the matrix, when they fall to the beach,
where, together with fragments of the schist, tney are gradually
reduced to pebbles and fine sand.

(4) Weathering influenced by Mineral Composition. Al-
though the soda-lime feldspars yield to the decomposing agen-

PLATE 20

FIG. 1. Weathered schist, coast of Cape Elizabeth, Maine.
FIG. 2. Sandstone bored by bees. FIG. 3. Slab of glaciated limestone.

WEATHERING INFLUENCED BY COMPOSITION 235

cies more readily than the potash varieties, basic eruptives do
not in all cases decompose more rapidly than the granitic rocks
into which they are intruded, as is well illustrated in some of the
glaciated areas about Boston, where small, compact dikes form
low ridges a few inches above the surface of the enclosing granite.
Much depends upon the grain of the rock and the character of
the secondary minerals which have been generated at some
period prior to its decomposition proper. Thus those dikes con-
taining so large a proportion of secondary epidote as to be of a
dull greenish hue are almost invariably more enduring than the
granites, while those on the other hand, in which the secondary
minerals are largely chlorite, calcite, and zeolitic compounds,
yield to the decomposing agencies more readily. Even when the
dike as a whole gives way, the presence of epidotic aggregates
frequently manifests itself in protruding knots and bunches
above the corroded surface. Knots caused by segregations of
black tourmalines stand out in the same way from the surface
of Stone Mountain, already referred to. Garnets, staurolites,
quartz veins, and other of the less easily decomposed minerals
may stand out in like manner from the surface of the rocks
of which they form a part.

Granitic and other complex crystalline granular rocks will, on
exposure, sometimes take on a pitted surface, owing to the re-
moval of the more easily decomposed materials. The boulders
of nepheline syenite in the glacial drift about Portland, Maine,
are thus corroded to the depth of several millimetres through
the removal of the granular nepheline, while the feldspars and
hornblendes project irregularly.

Calcareous rocks containing silicates, like the amphiboles or
pyroxenes, show like roughened surfaces due to the dissolving
away of the calcareous matter, leaving the silicates projecting
(Fig. 2, PL 14), or, as is the case with some of the tremolite-
bearing dolomites used for building, may become pitted by the
dropping out of the tremolite as the calcareous cement gives way. 1

Many sandstones become likewise roughened through the re-
moval of a portion of the cementing constituent, leaving the
siliceous granules projecting. In the coarsely crystalline lime-
stones and dolomites the solution and weathering effects are
often first manifested along cleavage lines and the contacts of

*As in the U. S. Capitol Building at Washington.

236 THE PHYSICAL MANIFESTATIONS OF WEATHEKING

the individual granules, as may be observed in many an old
tombstone or polished column.

Even where the decomposition is almost purely chemical, the
corroded surfaces are peculiarly irregular, as shown in PI. 14.
This feature is doubtless due to some imperceptible difference in
the texture of the stone, or to the presence of joints and flaws
which give direction to the solvent fluids. Prof. C. H. Smyth
has called the author's attention to the fact that in crystalline
limestones weathering commonly proceeds most rapidly along
cleavage lines and the JR twinning planes. Calcareous rocks
consisting of an admixture of calcite and dolomite crystals may
undergo disintegration through a complete or partial removal of
the calcite granules by solution, the dolomite remaining almost
untouched. Certain dolomitic limestones near Stockton, Min-
nesota, have been described 1 as peculiarly subject to this form
of disintegration. The mass of the rock consists of dolomitic
crystals and granules, but often interlaminated with narrow
bands of calcite. Through the removal of the latter, the
stone becomes porous and its degeneration so complete that
"shovelfuls of loose sand consisting of dolomitic rhombohedra
can be taken up. ' '

Fine-grained, compact, and seemingly homogeneous rocks
may, on account of imperceptible differences in composition
and structure, weather out in strikingly irregular and peculiar
forms. Compact limestones and other rocks losing materials
chiefly by solution sometimes give rise to markings so closely
resembling hieroglyphic or cuneiform characters, that it is not
surprising they have more than once been mistaken for the work
of human hands.

Massive granitic rocks seemingly of quite uniform composi-
tion will sometimes weather very irregularly, giving rise to
oven-like cavities, in general shape resembling the pot-holes in
the beds of streams. Reusch has described 2 such in exposed
faces of granite ledges on the island of Corsica, the holes
extending inward horizontally, or sometimes with an upward
tendency. The cause of this is not apparent from the descrip-
tion given, but it is presumably due to slight textural differences
such as are not readily discernible in the decomposed rock. 3

1 Hall and Sardeson, Bull. Geol. Soc. of America, Vol. VI, 1895, p. 184.

2 Forhandlinger i Videnskabs-Selskabet i Christiania, 1878, No. 7, pp.
24-27.

8 These cavities have since been described and figured by F. F. Tuckett
and T. G. Bonney (Geol. Mag., Vol. I, 1904) but no satisfactory explana-

WEATHERING INFLUENCED BY COMPOSITION 237

In any rock consisting of a variety of minerals, disintegration
is likely to constitute a more prominent feature of weathering
than in one of less complexity of composition, owing to the
unequally refractory properties of its constituents. Thus a
granite must yield a sand, while a purely feldspathic, pyrox-
enic, or calcareous rock may yield only clays.

Beds of feldspathic quartzite, through the decomposition of
the feldspar, undergo disintegration, giving rise to beds of
friable siliceous sand interlaminated with kaolin, as described by
Dana. 1 The same author also describes an interesting pseudo-
breccia formed by a quartzite divided up by a succession of
cracks into which limonite from decomposing pyrite has fil-
tered and acted as a colored cement. He says: "Many of the
pieces lie in place barely separated from one another, and ap-
pear to be undergoing new divisions. But in the lower part,
large pieces look as if there had been wide displacements; yet
the hardly disturbed condition of the upper half proves that
the apparent displacement is due to the extension of the color-
ing and penetrating limonite. The cracks are made in part
by the extremely slow, wedge-like action of the depositing
limonite. ' '

Heusser and Claraz 2 describe somewhat similar breccias
formed in Brazil through the weathering of crystalline schists
rich in iron. These breccias consist of angular fragments of
schist, more or less decomposed, firmly cemented by limonite.

The boulders of Oriskany quartzite in the Cretaceous gravel
about Washington, District of Columbia, are composed of
rounded and angular quartz fragments tightly bound together
by a fine granular crystalline aggregate of quartz and feldspar.
Disintegration first manifests itself on the exterior of the
boulders in the form of an irregular network of grooves or
channels, which gradually become more and more conspicuous
until the boulder falls into bluntly pyramidal fragments and
finally into sand. The microscope shows that the disintegra-
tion is due to the disaggregation and partial kaolinization of the

tion of their origin offered. Similar cavities have been also described in
Madagascar by the Rev. Barron who ascribes their formation to the pres-
ence of imprisoned vapors in the original rock magma; in other words,
to be comparable to the vesicular cavities in lavas.

'Am. Jour, of Science, Vol. XXVIII, 1884.

2 Ann. des Mines, 5th Series, Vol. XVII, 1860, p. 290.

238 THE PHYSICAL MANIFESTATIONS OF WEATHEBING

binding constituents whereby all cohesion is lost, and disinte-
gration follows from necessity. (Fig. 1, PL 22.)

This form of disintegration seems to take place only in boulders
exposed at or near the surface, and is believed to be due pri-
marily to expansion and contraction from alternations of tem-
perature.

Many rocks, owing to a lack of homogeneity, weather with
extreme irregularity and give rise to odd and sometimes fan-
tastic forms. In the case of a friable sand or limestone, sub-
ject to wind or rain erosion or to solution, certain portions may
be protected by a capping of other rock while the intervening
material is carried away. There thus arise spindle-shaped
forms of varying proportions, each capped by the roof or hat-
like block to which it owes its origin. Such have been noted
in many regions, and have been described by Hayden as occur-
ring on a colossal scale in Colorado.

(5) Results due to Position. In very many instances loose
blocks of stone lying exposed upon the ground will undergo
a more rapid disintegration from the lower surface, a feature
evidently due to the fact that this portion of the rock is kept
in a state of continual moisture. This form of disintegration
results in the production of oval, flattened, scale-like masses,
quite independent of the original jointing. In other cases
decomposition going on from all exposed sides of a joint block
may in time produce the so-called rocking-stones or "logans"
and "tors" of English writers, though some of these are un-
doubtedly nicely balanced boulders from the glacial drift.

A mass of rock may be prevented from undergoing disinte-
gration, even though partially decomposed, by its surroundings.
Thus, in driving the tunnel for the waterworks extension, in
Washington, natural joint blocks of hard and apparently firm
rock brought to the surface would fall away to loose sand in
course of a few days, or months, as the case might be, much
depending on the conditions of the weather and the state of
decay. This characteristic was sufficiently pronounced to attract
even the attention of the workmen, who described the rock as
" slaking" and believed it to contain quicklime.

The fact was that percolating waters had brought about a
partial kaolinization of the feldspar, and hydration, without
great oxidation of the iron-magnesian constituent. The original
pressure, coupled with that incidental to expansion from hydra-

PLATE 21

Weathered horizontally bedded Jurassic sandstones and underlying, thin-bedded
Calcareous rocks. Near Bluff City, Utah. U. S. G. S.

EESULTS DUE TO POSITION 239

tion, had, however, been sufficient to hold the mass intact until
exposed briefly to atmospheric influences.

The protective action of water, as sometimes shown in the
beds of streams and in deep ravines, may be only apparent, and
due to the fact that erosion exceeds decomposition, the stream
having cut its way down to fresh bed-rock. Professor Dana,
to be sure, writing more than half a century ago, 1 described the
basaltic rocks of Kiama, Australia, as in a condition of advanced
decomposition except where protected by sea-water. "It is a
general and important fact that a rock which alters rapidly when
exposed to the united action of air and water, is wholly un-
changed when immersed in water, or exposed to a constant wet-
ting by the surf." While no exception can be taken to the
conclusion regarding those rocks wholly immersed, the question
naturally arises in one's mind, if the absence of decomposition
products in those rocks constantly wetted by the surf and in
many stream beds may not be due, in part at least, to erosion, as
noted above. That rocks so situated are in a condition far from
fresh, is well known to any petrologist who has attempted to
gather specimens.

In the case of strata lying nearly horizontal, it rarely happens
that all possess the same power of resistance, the more friable
weathering away with the greatest rapidity, leaving the harder
layers for a time projecting in rib-like masses, to ultimately break
down in large angular blocks as the support below is gradually
removed. Friable beds of sedimentary rock are thus not infre-
quently protected by a capping of impervious lava. Continual
percolation of water through existing joints and fractures in
time, however, erode away, in part, the underlying material,
causing the landscape to assume the Table Mountain appear-
ance, where each flat-topped hill represents residual masses of
a once continuous plateau, now isolated in the manner described.

It is obvious that where a large series of sedimentary rocks
composed, it may be, of interbedded limestones, sandstones, and
argillites are turned up on edge and exposed alike to atmos-
pheric agencies, they will become eroded very unequally. If
chemical agencies alone prevail, the limestone will dwindle
away and perhaps give rise to long valleys or depressions
walled in by the more enduring sands and shales, and carry-
ing upon its bottom a fertile clayey soil representing not

1 Eeports of Wilkes's Exploring Expedition, Geology, p. 514.

240 THE PHYSICAL MANIFESTATIONS OF WEATHEEING

merely the insoluble impurities contained by the original lime-
stone, but also the mechanically disintegrated particles washed
in from the hills on either hand. This indeed may be consid-
ered the history of the fertile Shenandoah valley of Virginia,
famous alike for soft contours, beautiful scenery, and the exu-
berant fertility of its soils.

In cases where thinly bedded rocks lie sharply inclined, it
nearly always happens that certain layers decompose more read-
ily than others. There may thus arise strikingly ragged saw-
tooth contours, the more enduring layers standing out in sharply
serrate or wall-like masses, while the softer give way and be-
come obscured by their own debris.

When stratified rocks lie nearly or quite horizontally, much
must depend upon the character as regards permeability, etc.,
of the upper layers, since these may so protect the lower lying
as to retard or quite stop further disintegration. Further than
this, an easy and rapidly disintegrating superficial layer may
yield a residual clay so impervious as to protect the underlying
rocks as securely as a mass of rock itself, or so hard and tough
as to put a stop to purely mechanical erosion, as in the case of
the laterite beds of central India.

(6) Induration on Exposure. Many rocks, instead of becom-
ing disintegrated on exposure, undergo a kind of induration
upon the exposed surfaces. This is particularly the case with
some siliceous sandstones. The water with which the stone is
permeated holds in solution certain constituents, as silica, car-
bonate of lime, or iron oxides. When the rock is so situated
that this ''quarry water," as it is popularly called, is brought
to the surface and evaporated, it binds together the granules
composing the stone, forming thus a more or less superficial
coating of a more enduring nature. The induration sometimes
takes place so rapidly that even an exposure of but a few months
is sufficient to produce very marked results on freshly broken
surfaces. This peculiarity of certain classes of rocks has long
been known to quarrymen and stone workers, who recognize
the fact that a well-seasoned stone yields much less readily under
the chisel than one that is newly quarried. 1

A somewhat similar induration, due to purely superficial
causes, has been described 2 by Dr. M. E. Wadsworth, as taking

1 See Stones for Building and Decoration, p. 415.

2 Proc. Boston Soc. of Natural History, Vol. XXII, 1883, p. 202.

INDUE ATTON ON EXPOSUEE 241

place on the surface of exposed blocks of siliceous sandstone in
Wisconsin. "The St. Peters Sandstone is composed almost
wholly of a pure quartz sand, and in the outliers of it found on
the hilltops south of the town, the parts covered by the soil were
more or less friable, and the grains distinct; while the exposed
portions of the same blocks and slabs were greatly indurated,
the grains almost obliterated, and the rock possessed the con-
choidal fracture and other characteristics of a quartzite." In
this and other cases cited by Dr. Wadsworth, the cementing mat-
ter is silica.

The explanation given (in letter to the present writer) is to
the effect that all water, including that of rains, as well as ter-
restrial, dissolves silica, which is again deposited under suitable
conditions. Part of the silica apparently comes from the solu-
tion of the quartz, chalcedony, and opal, and a part from the
alteration and destruction of the silicates. Both solution and
deposition seem at times to take place on the immediate surface,
the interior waters in such cases playing no part.

P. Choffat regards it as possible that silica set free through
feldspathic decomposition in granitic rocks may, on evaporation,
be redeposited in an insoluble form in the interstices of the fresh
rock in the immediate vicinity, thus retarding if not wholly
preventing further decay in that direction. 1

Professor W. 0. Crosby, in a personal memorandum to the
writer, calls attention to the fact that in the disintegrated
granites of the Pikes Peak, Colorado, area, the rock is almost
invariably exceptionally firm and impervious along the joints,
indicating a local induration due perhaps to infiltration of iron
oxides or silica. Where a joint face bounds a ledge of rock, it
often maintains its integrity, weathering out in relief like a
quartz vein, while the granite is in a condition of advanced
degeneration all around. A slight break in the face of a joint
plane, in such cases, may lead to extensive disintegration behind
it, until it finally falls away from the disintegrating mass, a slab
of relatively sound rock.

Andesitic rocks in regions of limited rainfall have been noted
by Professor G. Vom Rath as having become covered on the
upper surface with a thin layer of brown iron oxide, which pro-

1 Sur quelques cas cl 'erosion atmospherique dans les garnites du Minho,
CommunicaQoes da Direc^ao Dos Trabalhos Geologicos de Portugal, Tome
3, Fasc. I, 1895-96, p. 17.
17

242 THE PHYSICAL MANIFESTATIONS OF WEATHEEING

tected them from further disintegration. Such crumbled away
only from the under surfaces, where they absorbed moisture from
the ground, and gave rise thus to peculiar tent-like and mush-
room-shaped forms.

The present writer has noted in the Madison valley, north
of the Yellowstone Park, rounded masses of a vesicular rhyolite
which have, through the same causes, been reduced to the con-
dition of mere shells with openings on the under side and that
facing the direction of the prevailing winds. In these cases,
however, the wind seemed to have aided in their formation, not
merely through transporting the disintegrated material, but by
catching up and whirling about the loosened granules within
the gradually enlarging cavity, where, by force of impact, as
already described, they become themselves agents of abrasion.
Some of the cavities observed were of sufficient size to afford
shelter for a human being and had served as temporary dens for
wild animals.

Eoth mentions 1 an induration evidently somewhat similar to
that described by Vom Rath above, as having taken place, on
the surface of a reddish yellow sandstone in Fezzan, North
Africa. The crust thus formed was so dense and hard as to
break with a shell-like fracture resembling basalt. A similar
incrustation on sandstone from the Lybian desert was found by
Zittel to consist of: manganese oxide, 30.57%; iron oxide,
36.86%; alumina, 8.91%; silica, 8.44%; barium oxide, 4.89%;
sulphuric acid, 4.06% ; phosphoric acid, 0.25% ; and water, 5.90%.

The Potsdam quartzites of Minnesota have had, in many in-
stances, an almost glass-like polish imparted to their exposed
surfaces through no other apparent agency than that of wind-
blown sand. Unlike a polish produced by artificial methods,
this wind polish extends to the bottoms of every little groove
and cavity, or over every protruding knob alike. In softer,
rocks, or rocks of less homogeneous structure, the same agencies
carve out the softer portions, leaving the more resisting pro-
truding, as already described on p. 163. This polish is so per-
fect, even on rough surfaces, as to suggest a partial solution of
the granules, and a redeposition of the dissolved matter in the
form of a glaze, but the microscope proves to the contrary.
The gloss is due wholly to superficial smoothing and no new

1 Allegemeine u. Chemische Geologie, 2d eel., Vol. Ill, p. 215.

INCIDENTAL COLOK CHANGES 243

matter has been deposited either on the surface or between the
granules.

(7) Changes in Color incidental to Weathering. That in
nearly every rock a change in color, the assumption of a
brownish or reddish hue, is an early indication of decomposition
has been made sufficiently apparent in the chapter devoted to a
discussion of the chemical changes involved. This discolor-
ation is, however, merely incidental, and not essential, and is
found to diminish, if not wholly disappear, as the distance from
the surface increases, as was noted in the case of the granites of
the District of Columbia (p. 186) and the diorites of the Sierra
Nevadas. (P. 262. See further under Color of Soils, p. 373.)

Granite and other highly feldspathic rocks carrying pro-
portionately small amounts of iron become almost invariably
bleached or whitened on the immediate surface, owing in part to
kaolinization and in part to the splitting up of the feldspars
along cleavage lines.

In extreme cases rocks consisting of an admixture of feldspars
and iron-bearing silicates, but in which the feldspar, owing to
its glassy nature, is quite inconspicuous, become almost snow-
white in the earlier stages of weathering. This as in the case
above mentioned, is due to the obscuring of the darker silicates
by the white product of kaolinization. Continued decomposition
must, however, attack the ferruginous constituent and the usual
staining ensue, unless, as in some cases possible, sufficient car-
bonic acid may exist to convert the iron immediately into car-
bonate and permit of its removal in solution.

Allusion has been already made to the fact that oxidation
or other chemical action, with. the possible exception of hydra-
tion, practically ceases below the permanent water level. Hunt
and Le Conte have both called attention to the fact that the
hornblendic and feldspathic rock fragments occurring in the
Pliocene auriferous gravels of California are firm and intact in
those portions below the drainage level (the blue gravel layer),
but more or less completely oxidized, kaolinized, and otherwise
altered in the red or upper gravel.

Van den Broeck has called attention 1 to the possibility that
the so-called red and gray diluvium of the Quaternary deposits
near Paris may be but portions of one and the same geological
body, the "diluvium rouge" being but an upper member of

1 Bull. Soc. Geologique de France, 5, 1876-77, p. 298.

244 THE PHYSICAL MANIFESTATIONS OF WEATHEEING

the "diluvium gres," oxidized and impoverished in lime by the
action of meteoric waters.

The same feature is noticeable in many of our quarries for
building stone, as those in the Berea sandstones of Ohio. The
beds below the drainage level, are of a gray or blue-gray color,
while above, where they have been subjected to the oxidizing
influence of meteoric waters, they are buff. The Jurassic oolites
of England, are blue-gray at some depths below the surface, but
white above.

In cases where natural joint blocks are exposed to the perco-
lation of meteoric waters, the weathering may for a time mani-
fest itself only in differential oxidation and zonal segregation
of the iron whereby are produced concentric bands of varying
hues. Fig. 3, PL 22, is a slab from a natural joint block of
argillite in the collections of the National Museum, in which
the bands, due to this cause, vary from yellow-brown, drab, to
ochreous yellow and red, while the rock as a whole still retains
its compact structure and susceptibility to polish, forming an
ornamental stone of no mean order. 1

W. P. Blake has described boulders from the Colorado desert
colored exteriorly by what he regarded as organic matter re-
ceived from water during a period of submergence. Similarly
discolored quartzite boulders brought by G. K. Gilbert from the
Sevier desert in Utah, and examined by the present writer, show
a thin dark varnish-like coating, not inaptly named by Mr.
Gilbert "desert varnish," and which consists largely of oxides
of iron and manganese, though a slight amount of organic
matter is present. In this case the rock is composed not wholly
of quartz granules, but carries interstitial calcite and feldspathic
granules. Near the discolored surface of the boulders these in-
terstitial calcites are found quite dissolved away, leaving cavities
stained by a dark deposit which reacts for iron and manganese.
Inasmuch as acid solutions obtained from fresh and uncolored
portions of the boulders give faint reactions of the same nature,
it seems very probable that the crust is due to a concentration
of these metals in a condition of higher oxidation on the surface,
whither they have been brought by capillarity, while the more
soluble lime carbonate was removed.' 2 It is freely acknowledged,

1 Stones for Building and Decoration, p. 169.

2 Although such discolorations seem to have been noted principally in
desert regions, they are by no means limited thereto. The quartzitic boulders

PLATE 22

FIG. 1. Weathered boulder of Oriskany sandstone.

FIG. 2. Concentric weathering in diabase.

FIG. 3. Zonal structure in weathered argillite.

FIG. 4. Weathered sandstone, showing induration along joint planes.

AMOUNT OF MATERIAL REMOVED IN SOLUTION 245

however, that this conclusion is not wholly satisfactory, since, if
correct, the intensity and depth of the colored zone should, to a
certain extent, be governed by the size of the pebble. Neither
are the conclusions in harmony with those of Walther, 1 who
found in the Egyptian deserts a superficial discoloration and
induration common to all classes of rocks, quite independent of
iron and manganese as original constituents, and due, as he
believed, to prevailing climatic conditions and the prevalence of
a certain amount of silica. The more highly siliceous the rocks,
the darker the colors of the crust. Limestones he found to turn
light to dark yellow, sandstone and siliceous dolomite dark
brown; many granites, jaspers and flints, black; all, as a rule,
being most highly colored on the surfaces exposed to the at-
mosphere.

No claim is made by Walther of a complete solution of the
problem, though he regards the Egyptian occurrences as cer-
tainly not due to the solvent action of water, and the fact that
like crusts are found in pebbles on the banks of tropical rivers
is considered as having no bearing on the cases described. It
would, to the writer at least, seem probable that phenomena of
a somewhat similar character, so far as appearances go, but due
perhaps to quite different causes, have been confused by the
various writers on the subject. Attention, before leaving the
subject, may be called to the fact that the darkest colors described
by Walther, were on rocks which from their nature would
weather away the most slowly. A dark color on the surface of
a granite or quartzite pebble may be due in part to the fact that
the immediate surface of a rock of this nature has been longer
exposed than that of a limestone which is continually losing by
solution or abrasion.

(8) Relative Amount of Material removed in Solution.
Among siliceous rocks, chemical action proceeds but slowly,
and the amount of material actually removed in solution is
rarely over 60%, and may be so small that, as the writer has
shown, 2 the residue in extreme cases occupies some 80% more
space than the rock from whence it was derived. Carbonate
of lime, the essential constituent of ordinary limestone, is,

in the superficial deposits of the District of Columbia show at times a like
discoloration, due to a very thin coating of iron and manganese oxide.

1 Denudation in der Wiiste, p. 117.

2 Bull. Geol. Soc. of America, Vol. VI, 1895, pp. 321-332.

246 THE PHYSICAL MANIFESTATIONS OF WEATHEKING

however, as has been observed, soluble in the carbonated water
of rainfalls, and, in time, may undergo complete removal,
leaving but the insoluble impurities behind. This is, indeed,
the almost universal history of limestone soils. They are not
infrequently so siliceous or ferruginous as to be quite barren
and of a nature to be benefited by the application of lime as a
manure.

Throughout the areas occupied by the Trenton limestones, in
Maryland, nearly every farm has, in years past, had its quarry
and lime-kiln where the stone was fitted for supplying lime
once more to soils from which it had been so thoroughly leached
as to render them lean and poor. It is to this solvent action
that is due the formation of the multitudinous caverns, large
and small, of the limestone regions. Even where caverns are
not apparent, the corrosive action is evident to the practised
eye. In the quarry regions of Tennessee surface blocks of
limestone are often grooved to a depth of an inch or more
with wonderful sharpness, simply from the water of rainfalls
with its acids absorbed from the atmosphere and surface soils,
while in the quarry bed the stone is found no longer in con-
tinuous layers, but in disconnected boulder-like masses. (Fig.
2, PL 23, and Fig. 3, pi. 14. ) x In such cases casual examinations
give very little clew to the rapidity of the destruction going
steadily on, since all is removed in solution excepting the com-
paratively small amount of insoluble matter (usually clay or
silica) existing as an impurity.

(9) Incidental Surface Contours. In limestone regions the
solvent action of water has frequently gone on so extensively
as to leave its imprint upon the topographic features of the
landscape. Narrow, symmetrical valleys, due wholly to solution,
have been described 2 in what is known as the Boone chert region
of northern Arkansas. Such have steep slopes and are of great
length in proportion to their width. In many a limestone area
the drainage is no longer wholly superficial, but by subterranean
streams sinking entirely into the ground to reappear again at
lower levels, it may be miles away, having traversed the inter-
vening distance in some of the numerous passages (fissures en-
larged by solution) with which the rocks abound. Entire land-

1 These are evidently identified with the so-called "karren" forms of
German writers. See Globus, Vol. 70, No. 7, August, 1896.

2 By Messrs. Marbut and Perdue, Jour, of Geology, 1901, p. 47.

F THl

UNIVERSITY

PLATE 23

FIG. 1. Sink-hole near Knoxville, Tennessee.

FIG. 2. Beds of marble corroded by meteoric waters, Pickens County, Georgia.

INCIDENTAL SUKFACE CONTOUES 247

scapes are undulating through the abundance of sink-holes
shallow depressions down through which the water has percolated
and escaped into the underground passages.

The writer recalls a beautiful illustration of this nature seen
in the limestone regions of southern Indiana, some years ago.
The season was that of the wheat harvest. On every side, far as
the eye could reach, were undulating fields of waving grain, of
that charming golden hue of which poets sing, with intervening
patches of woodland. From every farm was heard the click of
the reaper, and from every fence the whistle of the "Bob
White." Owing to the fact that the ridges between these de-
pressions were drier than the bottoms, the wheat here ripened
earlier, and field after field showed long reaches of saucer-
shaped depressions green in the centre, with intervening ridges
of golden brown, making, with that charming hazy atmosphere,
a picture long to be remembered. Through accident or design,
the opening in the bottom of these sink-holes sometimes becomes
closed, giving rise thus to temporary pools, or ponds, as shown
in the accompanying plate. (Fig. 1, PL 23.) It is this same
action that has given rise to the so-called "sandpipes" of the
English geologists. These are slender funnel- or tube-shaped
cavities found in chalk, and calcareous sandstone, sometimes filled
with drift gravels, sands, brick-earths, or again with fragmental
materials fallen into them from the overlying beds as the sup-
port beneath was gradually removed. In all these cases it is
assumed that direction was given the percolating water by pre-
existing fissures or lines of weakness. 1

In regions underlaid by massive siliceous crystalline rocks,
and where mechanical erosion is reduced to a minimum, land-
scapes are softly undulating, with few abrupt escarpments or
precipitous ledges, owing to the uniform rotting away of the
materials, and the gradual accumulation of the debris. It is to
this form of weathering that is due the beautiful rolling hills
of southwestern Maryland. The prevailing rock is granite or
gneiss. Decomposition follows out each line of weakness.
Streams erode through the softened material down to hard
bed-rock, while the relatively large proportion of insoluble
debris is left to accumulate on the gentle slopes which form
such an enchanting feature of these landscapes.

1 See Prestwich, Quarterly Journal Geological Society of London, 1855,
p. 62.

248 THE PHYSICAL MANIFESTATIONS OF WEATHERING

In regions of gneissic or granitoid rocks traversed by large
veins of quartz, as in the northwestern part of the District of
Columbia, the superior resisting power of the quartz causes it
to stand out in relief from the gradually dwindling rock masses
on either hand, giving rise thus to prominent knolls, or ridges,
the occasion for which is a mystery until we come to examine
the foundation materials. Belt, in describing the auriferous
quartz lodes at San Domingo, 1 states that the prevailing trend
of the main ranges is nearly east and west, and is probably due
to the direction of the outcrops of the lodes which have resisted
the action of the elements better than the soft dolerites.

So striking a feature of the landscape as the Devil's Tower
or Bear Lodge on Little Sun Dance River, Wyoming, is due to
the weathering away and erosion of sedimentary beds from
around a dense crystalline core or plug of eruptive rock in-
truded into them in some past period of volcanic activity.
Through its greater powers of resistance, this still stands,
towering over 1000 feet above the level of the river, though in
time this, too, must go. Quite similar forms have resulted,
within a comparatively brief geological period through the
erosion of tufaceous cones from around the compact, crystalline
plugs of lava which solidified within the crater when volcanic
activity ceased. Beautiful examples of these are to be seen in
Arizona and New Mexico, where they are known as "volcanic
necks." The formation of bosses through the influence of
joint planes has been described elsewhere (p. 231).

In regions abounding in intrusive olivine or pyroxene rocks
which have undergone alteration into serpentine and talc or
"soapstone," one frequently finds these materials forming the
main mass of the hills, while the valleys are carved out of the
softer, more readily decomposed granite, or whatever the country
rocks may be. The same feature is prominently developed in
the slate regions of Harford County, Maryland, where the slate
is the more enduring rock, and forms steep ridges, flanked by
valleys, carved out from less resisting materials. Regions of
trappean dikes in siliceous schists or gneisses, particularly
along sea-shores where swept by incoming tides, are often
characterized by narrow, straight-walled chasms, or canons due
to the weathering out of the basic rocks, while the more refrac-
tory schists on either hand remain.

ir The Naturalist in Nicaragua.

EFFACEMENT OF ORIGINAL CHARACTERISTICS 249

In cases where' trappean dikes have cut through friable sand-
stones, they have in some instances so indurated these rocks
along either contact as to cause them to be more durable than
the original rock or than even the trappean rock itself. There
may thus arise long parallel ridges of indurated sandstone sepa-
rated by an intervening depression due to the weathering out
of the dike material.

In regions where climatic conditions or the nature of the rock
are more favorable to mechanical disintegration than chemical
decomposition, contours may be ragged in the extreme. Entire
crests may be but successions of jagged peaks and intervening
narrow valleys which are gradually becoming choked up by the
debris fallen from the cliffs above.

(10) Effacement of Original Characteristics through Weath-
ering. In cases of extreme decomposition, in place, the residual
products may so slightly resemble the parent rock as to give rise
to very conflicting opinions concerning their origin. This was
for a long time the case with the laterite of India, already
described, and the terra rossa of Europe.

Dana describes 1 an interesting case of basaltic decomposition
which, on account of the peculiar nature of the residual product,
is worthy of mention here. He writes : ' ' The process of decom-
position is finely exhibited on the second cliff north of Kiama
(Australia) towards the north end. At first sight, a distinct
argillaceous deposit was supposed to overlie the columnar basalt ;
for it was twenty feet thick, and of a whitish color, resembling
a soft crumbling marl, thus wholly unlike the basalt, and the
common results of basaltic decomposition. Still it had pro-
ceeded from the alteration of a regular columnar variety, having
a dull grayish blue color. The original rock is exceedingly
compact, showing no trace of crystallization, excepting an oc-
casional minute crystal of feldspar; and within the reach of the
swell, it was still compact and solid.

"The rock has a concentric structure, and to this it owes in
part its rapid decomposition. The alteration commences be-
tween the concentric layers, rendering them apparent, although
not so before. At first a thin ochreous line appears, arising
from iron; either magnetic iron disseminated in the rock, or
from that of the constituent mineral augite. This ochreous
color afterwards mostly disappears, and the concentric coats

1 Reports Wilkes's Exploring Expedition, Geology.

250 THE PHYSICAL MANIFESTATIONS OF WEATHEEING

become separated by thin clayey layers of a white color, more
or less striped with ochreous lines. In a more advanced stage
of the process large ovoidal masses of basalt (but little changed
in appearance excepting the development of a slaty concentric
structure) lie in the cliff separated by a considerable thickness
of the whitish clayey layers, which are stained by irregular
ochreous lines At last the centres of the spheroidal masses
yield, and finally the change is so complete that the concentric
arrangement is entirely lost, and a soft whitish or yellowish-
white argillaceous deposit, with few ochreous spots or lines,
takes the place of the compact basalt.

" In basalts of more compact structure these changes take
place more slowly. The grayish blue basalt in the Illawarra
range, near Broughton's Head, when long exposed, is discolored
exteriorly to a depth of an inch and a half. The colors, begin-
ning within, are dirt-brown, grayish yellow, ochre-yellow,
brownish red; and they are evidently dependent mostly on
changes in the condition of the iron which the rock or its
minerals contain.

"When the rock includes much chrysolite, the results of
decomposition in some instances give a fissile or micaceous
appearance to the rock. At Prospect Hill, five miles west of
Paramatta, this change is in progress. The rock is a black
ferruginous basalt of homogeneous aspect, breaking with a
smooth fracture and no appearance of crystallization. It con-
tains chrysolite; but the grains are small and not apparent
except on very close examination. . . .

"Were we unable to trace the transitions, and distinguish
the columnar structure through the whole, we should scarcely
suspect its basaltic origin. Indeed, it was pointed out to me
as an instance of mica slate overlying basalt. Particles of
rusted mica, as they seemed, were distinct, and it much re-
sembled a decomposing variety of that rock. On close inspec-
tion and an examination of the rock in different stages of
change, it became evident that the pseudo-mica was nothing
but altered chrysolite, which had rusted from partial decompo-
sition, and split into thin cleavage scales.

"The crystals of chrysolite have evidently a parallel position
in the rock, and hence the plane of easiest cleavage lies in the
same direction, or, as the cleavage shows, parallel with the
upper surface, that is, at right angles with the vertical axis of

EFFACEMENT OF ORIGINAL CHARACTERISTICS 251

the columns. The passage from the compact to the decomposed
rock is, in this case, unusually abrupt. Alteration takes place
(through the elimination of oxide of iron as before suggested)
slowly at the surface, which therefore chips off as soon as de-
composed and exposes a new portion. This sudden transition
may, in part, proceed from the absence of any natural planes of
fracture (which are brought out when there is a concentric
structure), and perhaps in part also from the presence of
chrysolite. The layer of pseudo-mica schist is in some places
five feet thick and has a rusty brownish color. Above it passes
into three feet of earth of the same origin, having a brownish
black color, and this is covered again by four feet of brownish
red soil."

Such an effacement is not, however, an invariable accom-
paniment of decomposition, since where the amount of residuary
material is relatively large, and allowed to accumulate in place,
the mass may for a long period retain its original structural char-
acteristics. Indeed, the original features are sometimes so per-
fectly preserved that casual inspection alone quite fails to reveal
the havoc that has gone on. Every detail of bedding, jointing,
or foliation, or even of internal structure, as brought about by
the arrangement or size of the individual particles, may be re-
tained with perhaps only a slight change of color due to oxida-
tion. This feature is often strikingly conspicuous in the newer
railway cuts of the southern Appalachian regions, particularly
where the country rock is of the nature of gneisses or schists.
In the work of grading the streets, in the extensions of the city
of Washington, masses of strongly foliated granites, so soft as
to be readily removed with pick and shovel, would be cut
through, which yet showed every vein or other structural detail
as plainly marked as in the original rock, and it was only when
by thrusting one's cane or other implement into it that its thor-
oughly decomposed condition became apparent. Kussell de-
scribes 1 a similar condition of affairs prevailing in the coarse
Triassic conglomerate near Wadesborough, North Carolina.
This conglomerate is here composed of rounded and angular
pebbles of talcose schist and other crystalline rocks. In the
fresh cuts along the line of the North Carolina railroad, every
detail of the original rock is brought out almost as sharply as in
the so-called Potomac marble phase of the same formations as

*Bull. 52, U. S. Geol. Survey, 1889.

252 THE PHYSICAL MANIFESTATIONS OF WEATHEEING

used in the Capitol building at Washington. ''On examining
more closely, however, one is surprised to find that it is com-
pletely decomposed, and that when moist it can be cut with a
pocket knife through pebbles and matrix alike, as easily as so
much potter's clay. The full depth of the alteration in this
instance is not revealed, but it extends more than 30 feet below
the surface without change in character."

W. B. Potter described 1 the feldspar porphyry of Iron Moun-
tain, Missouri, as decomposed to the extent that it can be easily
whittled away with a penknife or scratched with the thumb nail.
"At the same time," he writes, "the original porphyritic
structure of the individual crystals scattered through the mass
is beautifully preserved, and is even frequently more distinctly
visible than in the original rock, owing to stronger contrasts of
color in the kaolinized material. ' ' In many dense massive rocks,
indeed, such features as flow structure and inequalities of text-
ure are rendered evident only on weathered surfaces. The same
is often true of fossiliferous limestones, a weathered surface re-
vealing the presence of organic forms wholly imperceptible on
one freshly broken.

The crude kaolin as removed from the pits near Brandy wine
Summit, Pennsylvania, and at Hockessin, Delaware, still retains
more are less distinctly the structure of the original gneiss or con-
glomerate from whence it was derived. The quartz granules
of the gneiss are, in these cases, almost invariably shattered,
as though crushed by dynamic agencies, and show distinctly
corroded surfaces, presumably caused by the alkaline carbo-
nates formed during the kaolinizing of the feldspars. The
black mica makes its former presence known by rust-colored
spots which, in those cases where the mineral was sufficiently
abundant, have ruined the material for the purposes of the
potter.

(11) Simplification of Chemical Compounds, incidental to
Weathering. It has been noted on p. 150 that the process of
weathering is but an attempt on the part of the elements in
their various combinations to adjust themselves to existing con-
ditions. This adjustment consists in the formation of new com-
pounds which are characterized by a less complex structure than
those first formed.

Indeed, one of the most striking features of chemical geology

1 Jour. U. S. Assoc. Charcoal Iron Workers, Vol. VI, p. 25.

KESULTS INCIDENTAL TO DECOMPOSITION 253

is the tendency toward simplification in composition as mani-
fested all over the superficial portions of the earth. During
the process of decomposition there is a constant breaking down
of complex molecules of mixed silicates of alumina, iron, lime,
magnesia, and the alkalies, and a recombination of their various
elements as simpler silicates, carbonates, sulphates, and oxides.

The production of carbonates, particularly those of lime,
is one of the most conspicuous results of rock weathering, and
according to Van Hise 1 is a matter of paramount importance.
Through the decomposition of lime-bearing silicates, as certain
of the feldspars, pyroxene and amphiboles, the lime separates
out as calcite or aragonite, as may be readily shown by micro-
scopic examinations or chemical tests.

(12) Other Results incidental to Decomposition and Erosion.
- That all the minerals of a rock mass are not equally acted upon
by atmospheric agencies has been sufficiently noted in previous
pages. The more refractory, freed by the breaking down of
their host, remain to gradually accumulate in vastly greater
proportions than they existed in the original rock. If, in
addition to their refractory qualities, such possess, as is usually
the case, greater density, decomposition and erosion may act but
as agents of concentration, and in such residues minerals like
xenotime and monazite have been found in abundance, although
occurring so sparingly in the fresh rock that their existence was
scarcely suspected.

It is in this manner that has originated the gem sand of
Ceylon. Precious stones have been found disseminated in limited
numbers in the granite converted into the cabook described on
p. 228. In weathering, the difficultly decomposable precious
stones have not been attacked, or attacked only to a limited ex-
tent. They have therefore retained their original form and hard-
ness. When in the course of thousands of years streams of water
have flowed over the layers of cabook, their soft, already half-
weathered constituents have been for the most part changed into
a fine mud, and as such washed away, while the hard gems have
only been inconsiderably rounded and little diminished in size.
The current of water therefore has not been able to wash them
far away from the place where they were originally embedded
in the rock, and we now find them collected in the gravel bed,
resting for the most part on the fundamental rock which the

1 Treatise of Metamorphisms, p. 479.

254 THE PHYSICAL MANIFESTATIONS OF WEATHEEING

stream has left behind, and which afterwards, when the water
has changed its course, has been again covered by new layers of
mud, clay, and sand. It is this gravel bed which the natives
call nellan, and from which they chiefly get their treasures of
precious stones. 1 The same process in states bordering along
the Appalachian Mountain system in North America has given
rise to auriferous sands, as well as to sands bearing monazite,
zircons, and other valuable minerals, which become segregated
merely through their greater density and power to resist decom-
position. The stream tin ores of the Malayan Peninsula, the
diamond-bearing gravels of Brazil, and indeed placer deposits in
general are illustrative of this same principle. The very soil
itself, although so indispensable to human existence, is but an
incidental and transitory phase of rock-weathering, as has been
made sufficiently apparent in previous pages. The deposits of
kaolin in western Pennsylvania and nothern Delaware, as else-
where noted, are but decomposed highly feldspathic gneisses
and conglomerates, while the phosphate deposits of middle Ten-
nessee are insoluble residue left by the leaching out of the cal-
cium carbonate from phosphatic limestones. 2

The Clinton iron ores of Alabama are, according to I. C.
Eussell, 3 insoluble residues left by the leaching out of the lime
from a ferriferous limestone. The same agencies that were
instrumental in bringing about the corrosion of the Shenandoah
limestones of Virginia transformed the disseminated sulphides
of zinc into carbonate and silicates and left them to accumulate
with the clay residue in the irregular pits and cavities with which
the surface abounds. Here again weathering has acted as a proc-
ess of concentration and rendered available ores originally too
widely disseminated to be of value. A perhaps still more im-
portant illustration is offered in the secondary enrichment of
ore bodies, and particularly those of copper, through a down-
ward leaching, by meteoric waters, and a redeposition of the
dissolved material at the permanent water level. It is by this
same leaching action on aluminous limestones that is formed the
so-called ' ' rottenstone " so commonly used in polishing brasses
and other metals.

1 Nordenskiold, Voyage of the Vega. See also Judd, On the Eubies of
Burma, etc., Philos. Trans. Eoyal Soc. of London, Vol. CLXXXVII, 1896,
p. 151.

2 J. M. Safford, American Geologist, October, 1896, p. 261.

3 Bull. 52, U. S. Geol. Survey.

THE WEATHERING OP BOCKS (Continued)
IV. TIME CONSIDERATIONS

Concerning the rate of decomposition of rocks of various
kinds, only very general rules can be laid down, since much
depends upon climatic conditions and the position of rock
masses relative to the action of frost, moisture, and the various
growing organisms.

(1) Rate of Weathering influenced by Texture. From the
study of building materials it has become apparent that a
coarsely crystalline rock will, all other conditions being the
same, disintegrate more rapidly than one of finer grain. This
is doubtless owing in part to expansion and contraction from
ordinary temperature variations, which act the more energetic-
ally the larger the crystalline particles. 1

It has already been remarked (ante, p. 40) that crystalline
rocks have a greater density than do glassy forms of the same
chemical composition. This indicates a contraction during the
processes of crystallization, which manifests itself, according to
at least one authority, in the development of minute interspaces
between the individual crystals. The coarser the crystalliza-
tion, then, the greater the amount of interstitial space, and
hence the greater the absorptive power.

1 The coefficient of cubical expansion for several of the more common
rock-forming minerals has been determined as follows:

Quartz 0.0000360 Tourmaline 0.000022

Orthoclase 0.0000170 Garnet 0.000025

Horfablende 0.0000284 Calcite 0.000020

Beryl 0.0000010 Dolomite 0.000035

The strain brought to bear upon a mass of rock through the unequal
rate of expansion of its various constituents is further complicated through
the unequal expansion of the individual minerals along the direction of
their various axes. Thus quartz gives a coefficient of 0.00000769 parallel
to the major axis, and of 0.000001385 at right angles thereto. Adularia
gives 0.0000156, 0.000000659, and 0.00000294 for its three axes, and horn-
blende 0.0000081, 0.00000084, and 0.0000095 (Stones for Building and
Decoration, p. 419).

255

256

TIME CONSIDEKATIONS

These coarser rocks, owing to their tendency to undergo a
mechanical disintegration, or disaggregation, may also yield to

the decomposing agencies
more readily than those
of finer grain, though
from the fact that they
first fall away to coarse
sand, whereby the rock-
like character is lost, one
might, on casual inspec-
tion, be led to the oppo-
site conclusion. It need
scarcely be said that,
among rocks having the
same composition, whe-
ther fragmental or crys-
talline, siliceous or cal-
careous, those of a granu-
lar structure will un-
dergo disintegration more
quickly than will those
in which the individual
minerals are closely com-
pacted or interknit, as in
many quartzites and dia-
bases.

(2) Rate of Weather-
ing influenced by Com-
position. Among rocks
of the same structure as
regards crystallization
and size of particles, the
basic varieties, such as the
diabases and gabbros, as

FIG. 20.

FIG. 21.

Microstructure of sandstone (Flo- 2f

* ig. &v

ing relatively large amount of interstitial
space and absorptive power, and (Fig. 21)
of diabase, with relatively little.

succum b more reac ]_
., ,,

lly than d the more acld
varieties like the granites.

This for the reason that the
iron-magnesian as well as the soda-lime minerals are more suscep-
tible than are the potash silicates and other essential constituents

KATE OF WEATHEKING 257

of the rocks of the granitic group. It is possible also that these
dark colors cause them to become more highly heated, where
exposed to direct sunlight, and hence subject to mechanical dis-
integration. The fact that many of our trappean rocks, as seen
in dikes cutting other rocks, do not in all cases succumb with
greater comparative rapidity is due to their very compact struc-
ture, whereby percolating waters are so largely excluded.

(3) Rate of Weathering influenced by Humidity. The ra-
pidity of rock weathering and soil formation is, even among
rocks of the same nature, widely variable, being dependent
upon climatic conditions of any particular locality. In the arid
regions north of Flagstaff, Arizona, are wide areas of country
covered with coal-black lapilli ejected from volcanoes whose
craters are now occupied by growing pines upwards of two
feet in diameter Yet these fields are, with the exception of the
pines, as bare of vegetation as though but yesterday scorched
by fire. The fine lapilli, resembling no-thing more than crushed
coke, cover everywhere the undulating plains, greedily absorb-
ing the moisture from melting snows and scanty rainfalls, but
undergoing no appreciable decomposition and affording foot-
hold for only a few desert shrubs and grasses. Yet in a
moister clime, and one more adapted for luxuriant vegetation,
we might expect that these lapilli should long ago have suc-
cumbed and given fairly fertile soils. (See further, p. 263.)

(4) Rate of Weathering influenced by Position. Among the
siliceous crystalline rocks superficial disintegration is undoubt-
edly greatly aided by temperature variations, which, by render-
ing the rocks porous, facilitate chemical decomposition. Such
action must, however, be merely superficial, and at considerable
depths below the surface the change must be purely chemical.
The chief conditions favoring chemical action are those of con-
tinual percolation by waters carrying carbonic acid, as already
described. It naturally follows, therefore, that a purely chem-
ical decay will progress more rapidly where the rock mass is
covered by such a layer of vegetable soil as shall keep the surface
moist and give rise to the decomposing solutions. Hence, that
such an accumulation having begun, decomposition will keep on
at an ever-increasing rate to a depth concerning which we have
at present no data for calculation. It must not be too hastily
assumed from this that rocks thus protected do in reality break
down more rapidly than those on bare hillsides, since, in the

258 TIME CONSIDERATIONS

latter case, where physical causes predominate, the loosened
particles are removed as fast as formed, and new surfaces for
attack are being continually exposed. Moreover, in assuming
that rocks decay rapidly where covered by vegetation, we must
not overlook the fact that the character of the overlying soil
may be such as to be protective rather than otherwise. Thus in
glaciated regions it is a well-known fact that the striae on rock
surfaces are found best preserved where they have been protected
from heat and frost by a mantle of drift, or the compact turf so
characteristic of the Northern states. (See further under In-
fluence of Forests, p. 266.) Culberson has noted 1 that rocks on
the southern slopes of hills in southeastern Indiana undergo a
more rapid weathering than those on the northern. This he
regards as due to the more frequent and more extreme changes
in temperature on the south slopes, which in that latitude receive
a larger amount of heat from the sun 's rays.

(5) Relative Rapidity of Weathering among Eruptive and
Sedimentary Rocks. As to the relative rapidity of chemical
decomposition among eruptive and sedimentary rocks, there
can with the exception of the calcareous varieties be no
question, the eruptives being far the more susceptible. This
for reasons which will be at once apparent when we consider
their origin. The eruptive rocks result from the comparatively
sudden cooling of magmas originating far below the action of
atmospheric agencies, and are pushed up and allowed to solidify
under conditions which are not at all conducive to chemical
equilibrium. They are compounds of elements which have
combined according to the conditions under which they tempo-
rarily existed, but which undergo continual changes as they
become exposed by erosion and other causes. They become, in
short, out of harmony with their surroundings, and there are at
once set up a series of physical and chemical changes such as
shall result in products more in harmony with existing condi-
tions, and hence more stable. These changes, briefly put, are
those involved in the weathering processes we have described.
Indeed, we may well say that rock weathering and all the seem-
ingly endless processes of rock decay and rock consolidation
are but stages in the continual efforts being made by these inor-
ganic particles to adjust themselves to existing conditions. But
. Indiana Acacl. of Science, 1879, p. 167.

EELAT1VE KAPIDITY OF WEATHERING 259

the sedimentary rocks (exclusive of the calcareous varieties) are
themselves the actual products of these adjustments. The con-
glomerates, sandstones, shales, and argillites are but the detrital
remains of eruptive rocks which under the various weathering
influences have become disintegrated and decomposed, their more
soluble constituents quite or in part removed, and the residues
laid down and consolidated under conditions such as to-day
exist upon or near the surface of the earth. They have, it is
true, been laid down under water; they are subaqueous, but
their decomposition and disintegration was subaerial. Hence,
when elevated above the ocean's level to become a part of the
dry land, they are for the most part comparatively stable, sub-
ject to only such chemical changes as oxidation, and it may be
dehydration. All other things being equal, then, those siliceous
rocks which are the product of mechanical sedimentation will be
found far less susceptible to the chemical action of the atmos-
phere and meteoric waters than are the eruptives. While they
may undergo a transformation into soils, it is mainly through
the disintegrating effects of heat and frost. Sedentary soils
resulting from such disintegration resemble, therefore, their
parent rock more than those of any other class.

Turning now to calcareous rocks, we shall find a quite differ-
ent state of affairs prevailing, owing to the different chemical
nature of the material and its ready solubility. These rocks
represent, in fact, the soluble portions of the eruptive rocks
which have been leached out during the process of decomposi-
tion. They are themselves solution products, although their
immediate deposition has been brought about through mechanical
agencies, as in the laying down of beds of shell marl upon a
sea-bottom. The lime leached out of terrestrial rocks is carried
in solution into the sea, where, taken up by molluscs and corals
as a carbonate, it becomes precipitated to the bottom on their
death, and may reappear as a limestone, or, if mixed with suffi-
cient quantities of other constituents, as a marl, calcareous
sandstone, or shale. Such on their re-elevation are still subject
to chemical change, owing to the ready solubility of lime car-
bonate in terrestrial waters, and so the endless round begins
once more. Reference has already been made to the amounts
of lime carbonate that may thus be annually removed from
the earth's surface, but one may add here, that, according to

260 TIME CONSIDEEATIONS

J. Gr. Goodchild, certain English limestones waste away, super-
ficially, at the rate of one inch in 300 years. 1

(6) Time Limit of Decay. We are sometimes enabled to
put a time limit on the beginnings of decomposition such as
shall enable us to gain at least a geological measure of the
rapidity of the process. This is the case with the disintegrated
granite of the District of Columbia described on p. 185. The
residual material is here now overlaid by clastic deposits of such
a nature as to force the conclusion that they were laid down by
water under such conditions as would have thoroughly eroded
away all underlying pre-existing decomposed material. It is
therefore inferred that this decomposition has taken place since
the clastic material was deposited, or, since these are of Creta-
ceous age, that it has taken place since the close of Cretaceous
times. In the same way, since glaciation must have carried
away the pre-existing disintegrated matter from the dike of
diabase at Medford, leaving the surface smooth and hard, so
here it is inferred that the decomposition is post-glacial. It is
but rarely that the rate of decomposition of any rock has been
sufficiently rapid since the beginning of human history, to be
of geological significance, though weathered surfaces in old
quarries, or the walls of old buildings offer abundant illustration
of what we might expect, could observation be extended over
whole geological periods instead of but a few years. It should
be remembered, however, that, in the latter case, the conditions
are quite different from those existing in nature, and the rate of
weathering may be accelerated or retarded, as the case may be.

Stone implements, made by prehistoric man, as now found
in graves, or dug from the soil, sometimes show incipient signs
of decomposition, as indicated, when broken across, by a change
in color and texture from without inward. Flint arrow and
spear-heads from prehistoric caves or mounds in Europe,
England, or America, often present on the outer surface a thin
crust or patine of a gray or white color extending inward, it
may be, for the distance of two or more millimeters. A grooved
stone axe of diorite found in eastern Massachusetts and now in
the collections of the National Museum at Washington, 2 shows
concentric exfoliation extending inward to a depth of from

1 Geological Magazine, 1890, p. 463.

2 Specimen No. 172,794, Archaeological Series.

TIME LIMIT OF DECAY

261

three to six millimetres, and comparable to that on the diabase
boulder figured on PL 22. It is of course possible that the axe
was made from a boulder, itself not quite fresh, but this seems
scarcely probable, and the inference is fair that both the patine
and the exfoliation are due wholly to weathering subsequent to
the manufacture of the implements on which they occur.

Mills 1 regards the extreme condition of decomposition exist-
ing in the Archaean rocks of Brazil as having taken place prior
to the deposition of the loess, that is, in the long interval between
the elevation of the Archaean rocks and the beginning of Qua-
ternary times. Inasmuch, however, as the Quaternary gravels
and loess are all readily permeable by water and not of a nature
to be themselves readily affected, it would seem possible that
at least a portion of the decomposition might have been brought
about since their deposition and, indeed, be still in progress.

The writer is informed by Mr. W. Lindgren that the granitic
diorites of the Sierra Nevadas of California, which are of late

FIG. 22. Flint implement showing weathered surface.

Jurassic or early Cretaceous age, are often decomposed and dis-
integrated to a maximum depth of 200 feet, the extreme upper,
1 American Geologist, June, 1889, p. 345.

262 TIME CONSIDEEATIONS

more superficial portions being reduced to the condition of a
red clay, while the lower are merely rendered soft and friable,
with little if any change in color. This disintegration has gone
on to such an extent that where the rock is traversed, as is
sometimes the case, by numerous gold-bearing quartz veins, the
entire mass of material is washed down by water hydraulicked
as in the ordinary process of placer mining. The Pliocene
andesites are also in places decomposed to a depth of 20 feet.
The region is one of heavy annual precipitation, but the rain-
fall is limited almost wholly to the winter season.

Rock disintegration and decomposition, after the manner
already described, have been by no means limited to the present
era, but have been going on since the first land appeared above
the surface of the primeval ocean. The results of the recent
decomposition are more apparent, since the derived materials are
still recognizable as rock debris, while that formed in past ages
may have been so changed by the solvent and assorting power
of water, the chemical action of the atmosphere, and the general
agents of metamorphism, as to have quite lost its identity.

Dr. R. Bell, of the Canadian Geological Survey, has described 1
an interesting illustration of pre-Palaeozoic decay in the crystal-
line rocks north of Lake Huron. The red granite, where it has
been protected from glacial action, is found to be eaten into
hollows in the form of round and sack-like pits and small
caverns, the last-named generally occurring on steep slopes or
perpendicular faces of the rock. These pits are most usually on
sloping surfaces, and in places are of sufficient size to allow two
men to crouch within. The granite around these pits shows no
indications of decay. That they are of pre-Palseozoic origin is
demonstrated by the presence in them of residual patches of the
fossiliferous Black River limestone, which Professor Bell regards
as veritable inliers of the Black River formation once filling all
the inequalities and still overlying the granite at lower levels,
though elsewhere almost wholly removed by erosion. Figure 23,
after Bell, shows diagrammatically the old granitic corroded
floor upon which the calcareous sediments were laid down, with
pits *still containing residual masses of the limestone, and the
still intact beds passing under the waters of Lake Huron at the
lower right.

1 Bull. Geol. Soc. of America, Vol. V, 1894, pp. 35-37.

WEATHERING IN COLD AND WARM CLIMATES 263

Pumpelly, too, has shown 1 that the diabase dike at Stamford,
Massachusetts, had undergone extensive decomposition prior
to the deposition of the Cambrian conglomerates. Of equal
interest and still greater economic importance was the sugges-
tion by this same authority, subsequently abundantly confirmed
by W. B. Potter, 2 that beds of iron ore lying on the western
flank of Iron Mountain, Missouri, and covered by Silurian lime-
stones, were true detrital deposits resulting from the pre-Silurian
breaking down of the ore-bearing porphyry forming the mass

atony jouit p/a

shaped-

FIG. 23.

of the mountain. These and other* illustrations that might be
given point unmistakably to the identity of geological processes
and correspondence in results since the earliest times, even did
not analogy and the thousands of feet of secondary rocks furnish
us safe criteria upon which to base our inferences.

(7) Relative Rapidity of Weathering in Warm and Cold Cli-
mates. For many years an impression has prevailed to the
effect that rocks decomposed more rapidly in warm and moist
than in cold climates. While, owing to abundance of vegeta-
tion and other supposed favorable conditions, a more rapid

1 Ibid., Vol. II, 1891, p. 209.

2 Jour. U. S. Assoc. Charcoal Iron Workers, Vol. VI, p. 23.

8 See also T. Sterry Hunt, The Decay of Rocks Geologically Considered,
Am. Jour, of Science, Vol. XXVI, 1883, p. 190.

264 TIME CONSIDERATIONS

decomposition might be expected, such has not as yet been
proven to actually take place, and indeed many facts tend to
prove the impression quite erroneous. Lack of decomposition
products in high latitudes is frequently due to glaciation or
erosion by other means. Whitney, 1 Irving, 2 Chamberlain, and
Salisbury 3 have shown the presence of residual clays of all
thicknesses up to 25 feet in the driftless area of Wisconsin,
and Chamberlain has described 4 limited areas of strongly decom-
posed gneiss in the non-glaciated areas of Greenland.

Moreover, we have no actual proof that the action of frost
is, on the whole, protective, as is stated by Branner. 5 It must
be remembered that frost, excepting in the extreme north,
penetrates to but a slight depth, and while it undoubtedly puts
a temporary stop to chemical action on the immediate surface,
it remains yet to be shown that the mechanical disruption that
ensues, as described in previous pages, is not as efficacious
as would have been the chemical agencies alone, had they been
permitted to continue their work. Through bringing about a
finely fissile or pulverulent structure, whereby a vastly greater
amount of surface becomes exposed, frost prepares the way for
chemical action at a thousand-fold more rapid rate than could
otherwise have been possible. If, further, as the writer has
elsewhere at least suggested, 6 hydration is the most potent
factor in rock decomposition, the process can go on uninter-
ruptedly below the level of freezing.

Professor H. P. Gushing has described 7 the argillites in the
vicinity of Glacial Bay, Alaska, as in a condition of great dis-
integration, wholly through the action of frost. "Disintegra-
tion," he says, "takes place with amazing rapidity, as shown
by the enormous piles of morainic matter furnished to the tribu-
taries of Muir Glacier, whose valleys are adjoined by mountains
of argillite, and by the massive talus heaps that are rapidly
accumulating at the bases of other mountains made up of the
same material." In a private communication to the present

1 Eep. Geol. Survey of Wisconsin, 1861.
2 Trans. Wisconsin Acacl. of Science, Vol. Ill, 1875.
8 Ann. Eep. U. S. Geol. Survey, 1884-85, p. 254.
*Bull. Geol. Soc. of America, Vol. VI, 1895, p. 218.

5 Bull. Geol. Soc. of America, Vol. VII, 1896, p. 282.

6 Bull. Geol. Soc. of America, Vol. VI, 1895, p. 331.

7 Trans. N. Y. Academy of Science, Vol. XV, 1895.

RELATIVE RAPIDITY OF WEATHERING 265

writer, he further states that the diabases of the region are
fully as much decomposed as are those in the Adirondacks of
New York, and that the blocks of eruptive rocks occurring in
the moraines of Muir Glacier are far gone in decomposition.

Mr. C. W. Purrington has made similar observations, and
states 1 that on the south side of Silver Bow Basin, some three
miles west of Juneau, at an elevation of 2000 feet above sea-
level, he found schistose diorites disintegrated over a consider-
able area to a depth of 20 feet. The particular locality cited
was on a mountain slope, where landslides were frequent, and
other conditions prevailed such as would prevent the accumula-
tion of the debris throughout a prolonged geological period or
to a very great depth. There could be, however, no doubt as
to the residuary character of the material observed, and the
inference drawn was to the effect that the disintegration had
taken place within a comparatively brief space of time. G. E.
Culver has also described 2 a diabase dike in Minnehaha County,
South Dakota, an arid region lying within the glaciated area, as
decomposed throughout the whole exposures from its upper
surface down to a depth of 20 or 25 feet, the limit of disinte-
gration being the drainage level of the region as marked by
the bed of a stream cutting through it.

On the other hand, Professor I. C. Russell, who has devoted
much attention to the subject of rock-weathering in both high
and low latitudes, is of the opinion that rock decay is a direct
result of existing climatic conditions. He states that decay goes
on most rapidly in warm regions where there is an abundant
rainfall, and is scarcely at all manifest in arid and frigid
regions. 3 Professor Russell's observations are of more than ordi-
nary value, since he has discriminated between decay and dis-
integration, which most writers have failed to do.

Since climate is dependent upon altitude as well as latitude
the relative rapidity of weathering in mountain regions and those
near sea-level is worthy of consideration. The sharp contrasts
of temperatures on mountain peaks bring about excessive ex-
foliation and disintegration, as has been noted by every traveler
in high altitudes. Indeed it has been suggested, I believe by
Penck, that the actual average height of mountains is limited

1 Surface Geology of Alaska, Bull. Geol. Soc. of America, Vol. I, 1890.

2 Wisconsin Academy of Sciences, Art, and Literature, 1886-91, p. 206.

3 Personal Memoranda to the writer.

266 TIME CONSIDERATIONS

by the fact that disintegration increases so rapidly with altitude
that the rate of uplift may not exceed that of degradation.

Eelative to the subject of rock degeneration in temperate re-
gions, we have to consider the possible increased amounts of
atmospheric gases brought down by snowfalls, over those brought
by rain. The snowflakes so completely fill the air as to rob it of
a larger proportion of its impurities than would a corresponding
amount of precipitation in the form of rain. Further, the snow
in melting affords the water better facilities for soaking into the
ground than though the same amount was poured down during
the comparatively brief period of a shower. How far these
agencies may go toward counterbalancing the effects of the con-
tinued higher temperatures of the tropics, we have no means of
judging. 1

Influence of Forests. It is even questionable if decomposition
has actually gone on to greater depths in regions covered by
forests, as contended by Hartt 2 and Belt 3 than elsewhere. Indeed
observations by geologists of the Egyptian Survey* are to the
effect that rock degeneration has proceeded at a fairly rapid rate
in regions completely lacking in forest growth. The high granite
ridge, bordering on the Red Sea, is described as ' ' remarkable for
the number of sharp, ragged peaks it shows and bounded in
many cases by almost sheer precipices which are rendered in-
accessible on account of the rotten nature of the rock." The
accumulation of a large amount of organic matter is undoubtedly
favorable to decomposition, but the growing vegetation constantly
robs the soil beneath of moisture and other elements necessary
for its growth, storing it away in the form of woody fibre or
sending it off into the atmosphere once more. The amount of
moisture that a full-grown tree evaporates daily through its
leaves is simply enormous, and is often made conspicuously ap-
parent by the dry knolls that may be seen surrounding isolated
trees or groups of trees in swampy areas. Indeed, Mr. R. L.
Fulton, in discussing 5 the influence of forests in the mountain

1 There is an old saying among Eastern farmers to the effect that a late
spring snowstorm is as good as a dressing of manure. It undoubtedly arose
from an appreciation by the farmers of the fact that the snow was more
beneficial than rain for the reasons above mentioned.

2 Physical Geography and Geology of Brazil.

3 The Naturalist in Nicaragua, p. 86.

4 Geological Survey Report, Cairo, 1902, p. 62.
6 Science, April 10, 1896.

INFLUENCE OF FORESTS

267

regions of the West, states it as his belief that the local springs
and streams are ' * more diminished by the water used by the trees
than by evaporation in their absence. ' '

It has been shown 1 that the total amount of moisture returned
into the atmosphere from a forest by transpiration and evapora-
tion from the trees and underlying soil, is about 75% of the
total precipitation. For other forms of vegetation it varies
between 70% and 90%, the forest as a rule being surpassed by
the cereals, while the evaporation from a bare soil is but 30%
of the precipitation. To this should be added the fact that
the activity of evaporation from forested areas is continued
throughout a longer period of each year, as a rule, than in
non-forested, for the simple reason that the grasses and cereals
early ripen, and practically cease to exhale altogether. This
is particularly the case in cultivated areas and prairie regions.
Hence, while the daily evaporation from given areas might for
a time be nearly equal, the annual amount is likely to be greatest
for that which is forested.

Further, it has been shown that only 70% as much rainfall
reaches the soil in the woods as in the open fields, the rest
being caught in the leaves, branches, and trunks, whence it is
returned directly to the atmosphere by evaporation. These
percentages naturally vary with the character of the forest
growth. In this connection the following tables, showing the
measured amounts of water at varying depths in a loamy soil
under forests of spruce, twenty-five, sixty, and one hundred

WATER CONTENTS OF A LOAMY SAND; RESULTS BY SEASONS EXPRESSED IN
PERCENTAGES OF THE WEIGHT OF THE SOIL

SEASON

SPRUCE

25 YEARS OLD

60 YEARS OLD

16 inch

32 inch

Average

16 inch

32 inch

Average

Winter (January and February) .
Spring (March to May) ....
Summer (June to August) . . .
Fall (September to November) . .

20.23
18.62
15.10
16.57

17.00
18.02
16.22
17.57

18.61
18.32
15.96
17.07

18.06
15.29
14.42
13.49

17.76
16.28
17.03
16.52

17.91
15.78
15.72
15.00

1 See Bull. No. 7, Forestry Division, U. S. Dept. of Agriculture, 1893.

268

TIME CONSIDEEATIONS

and twenty years old, and one bare of all vegetation, are instruc-
tive. It will be observed that the average amount is appreciably
greater in the bare soil, and that the least amount is found
under forests 60 years old, when we may assume the trees are
in their prime.

SEASON

SPRUCE

NAKED SOIL

120 YEARS OLD

16 inch

82 inch

Average

16 inch

32 inch

Average

Winter (January and February) .
Spring (March to May) ....
Summer (June to August) . . .
Fall (September to November) . .

19.75
17.47
17.78
14.88

22.44
20.83
20.90
19.46

21.09
19.15
19.97
17.17

19.96
20.66
19.77
20.04

24.73
20.51
19.98
20.20

22.35
20.58
19.97
20.12

Other experiments have shown a marked difference in the
distribution of the water in the forest-covered and naked soils,
in the first-named a much larger proportion being held in the
extreme upper portion than in that which was unprotected.
This is a natural consequence of the absorptive properties of

AVERAGE OF WATER CAPACITY, EXPRESSED IN PERCENTAGES OF THE WEIGHT

OF THE SOIL

SPRUCE

UNSHADED

DEPTH

25 Years
Old

60 Years
Old

120 Years
Old

SOIL

to 2 inches

30.93%

29.48 %

40.32%

22.33 L

6 to 8 inches

19 19

18 99

19 30

20 62

12 to 14 inches . ...

19.10

16 07

18 28

20 54

19 to 20 inches

18.40

16 26

20 16

20 14

30 to 32 inches

17.91

17.88

21 11

20 54

the accumulated humus. The above table, as compiled by
Fernow 1 from the work of Ebermayer, illustrates this point.
It is obvious that it is only that portion of the water which
1 Bull. No. 7, Forestry Division, U. S. Dept. of Agriculture, 1893.

Ik WEATHERING IN COLD AND WAKM CLIMATES 269

passes through this superficial blanket of mould that can be
instrumental in promoting rock decomposition. Hence the
presence of such a blanket may exert a protective, or at least
conservative, rather than destructive action. Further than this,
we have to remember that plant growth tends to reduce the
extremes of temperature and, even more, to diminish evapora-
tion from the immediate surface. The constant action of gravity
and capillarity in pumping the water down and up through
the soil is therefore largely diminished. Since it is by tempera-
ture changes and water action that decomposition is so largely
brought about, it is apparent that one must not be too hasty in
assuming that forest action is actually destructive; it may be
largely conservative. It is probable that the apparent amount
of decomposition in wooded areas is due to protection from ero-
sion, and the consequent accumulation of the residuary material.
(8) Difference in Kind of Weathering in Cold and Warm
Climates. That there may be a difference in kind in the de-
generation in warm and cold climates, or at least in moist and
dry climates, is possible and even probable. 1 In cold and in
dry climates subject to extremes of temperature, as in the arctic
regions or in the arid regions of lower latitudes, the weathering
is at first almost wholly in the nature of disintegration, a process
of disaggregation whereby the rock is resolved into, first, a gravel
and ultimately a sand composed of the isolated mineral particles
which have suffered scarcely at all from decomposition. The
writer has elsewhere referred to this form of degeneration as
manifested in the desert regions of the Lower Californian penin-
sula. 2 In a warm, moist climate chemical decomposition may
or may not keep pace with the disintegration, according to local
conditions, so that the resultant material may be in the form of
an arkose sand, as in the District of Columbia, or a residual
clay, as in the more superficial portions of the residual deposits
to, the southward. In certain cases, or among certain classes of
rocks, the decomposition proceeds at so rapid a rate that there is
scarcely any apparent preliminary disintegration. Local cir-
cumstances and character of rock masses being the same, we are,

1 The majority of writers have failed to discriminate between decomposi-
tion and disintegration. That there may be a very marked difference, due
mainly to climatic conditions, is the point I wish to emphasize here. See
also Walthers, Denudation in der Wiiste, p. 22.

2 Bull. Geol. Soc. of America, Vol. V, 1894, p. 499.

270 TIME CONSIDEKATIONS

however, apparently safe in assuming that in warm and moist
climates decomposition follows so closely upon disintegration
as to form the more conspicuous feature of the phenomenon,
while in dry regions, or those subject to energetic frost action,
mechanical processes prevail and disintegration exceeds de-
composition.

Dr. Hugh Worth has noted 1 that the product of the weather-
ing of dolerite, in England, was not attended with any excessive
loss of silica and that 'the ultimate product was a ferruginous
clay, rather than a beauxite, or hydrargillite, as in India. The
same features are brought out in our own analyses. How uni-
versal or how dependent this difference may be on climate it is
yet too early to say.

Accepting the facts thus far given, there is at once suggested
the idea that the lithological nature of sedimentary rocks, as
well as their fossil contents, may be regarded as indicative of
prevalent climatic conditions.

The possibility of estimating these conditions by the char-
acter of the debris resulting from the degeneration of feld-
spathic rocks was first suggested by the geologists of the Indian
Survey, 2 the undecomposed feldspars in the Panchet (Mesozoic)
sandstones being regarded as indicating a recurrence of a cold
period during which mechanical forces preponderated over those
purely chemical. The same idea was subsequently put forth,
quite independently, by the present writer. 3 That rocks in arid
regions do actually undergo less decomposition during the
weathering processes is shown not only by the fresh character
of the residuary material. Judd has shown 4 that rivers like
the Nile, draining regions of great aridity, though in a con-
dition of high concentration from prolonged evaporation, carry,
in solution, smaller proportional amounts of derived salts than
do those of humid regions.

Russell has noted that in the Yukon River region of Alaska
disintegration so far exceeds decomposition that the talus from
the mountains composed of loose, angular masses of rock quite
free from vegetation, forms what he calls debris streams, which

1 Geol. Mag., Jan., 1904.

2 Geol. of India, 2d ed., Vol. I, p. 201.

a Bull. Geol. Soc. of America, Vol. \ II, p. 362.

4 Keport on Deposits of the Nile Delta, Proc. Eoyal Society of London,
Vol. XXXIX, -1885.

EATE OF WEATHERING 271

actually creep slowly down the slopes, the movement taking
place principally in the winter time and being due apparently
to the slow settling, or creep, of deep snows. He states it as
his opinion that the mountains of the region have suffered more
through this form of disintegration than have those of Colorado
or the southern Appalachians, but less than those of the Great
Basin area. The range of limestone mountains along the Yukon
is pictured as presenting a crest of sharp, blade-like crags, flanked
by vast slopes of loose, angular stones on either side, the rock
being everywhere fresh and undecomposed, but badly shattered
and fissured.

(9) Extent of Weathering. The depth to which weather-
ing has penetrated necessarily varies greatly. In cases where
the detrital material is removed nearly or quite as rapidly as
formed, it may go on indefinitely, until, it may be, thousands
of feet of material have melted away; where, however, remain-
ing in place, decomposition must be gradually retarded until a
time comes when it practically ceases. In the region about
Washington, District of Columbia, the writer has observed the
granitic rock so disintegrated at a depth of 80 feet from the
present surface as to be readily removed by pick and shovel.
Even greater depths have been noted by writers on the geology
of our own Southern states and Central and South America.
Spencer states 1 that in the region about Atlanta, Georgia, the
rocks are "completely rotted" to a depth of 95 feet, while
"incipient decay" may reach to a depth of 300 feet. W. B.
Potter describes 2 the feldspar porphyry of Iron Mountain in
Missouri as decomposed to a visible extent as far into the hill as
mining operations had been carried, while to depths varying from
10 to 80 feet the kaolinization is complete. C. W. Hayes has noted
that diorites in the Chattanooga district of Tennessee are often
weathered to the condition of incoherent sand for a distance of
from 50 to 75 or even 100 feet from the surface, 3 while Sterry
Hunt, as long ago as 1875, called attention 4 to the evident signs
of weathering in the rocks pierced by the Hoosac tunnel, in
Massachusetts, at a depth of 200 to 300 feet.

The coarse granite of Pikes Peak, Colorado, is reported as

Survey of Georgia, 1893.

2 Jour. U. S. Assoc. Charcoal Iron Workers, Vol. VI, p. 25.

3 Nineteenth Ann. Rep. U. S. Geol. Survey, 1897-98, Part II, p. 18.

4 Trans. Am. Inst. of Min. Engs., Vol. Ill, 1875.

272 TIME CONSIDERATIONS

disintegrated to a depth of from 20 to 30 feet. Belt 1 describes
dolerite in Nicaragua, as shown by steep cuttings in mines, de-
composed to a depth of 200 feet. "Next the surface," he says,
"they were often as soft as alluvial clay, and might be cut with
a spade."

Derby describes 2 certain shales in Rio Grande do Sul, Brazil,
reduced by decomposition to the condition of reddish, drab, green-
ish, black, and umber-colored clays to the depth of 120 metres
(394 feet), and W. H. Furlonge has described 3 the granite of
the Dekaap gold fields, in the Transvaal, South Africa, as de-
composed to a depth of 200 feet. Rain erosion has carved out
from this decomposed mass deep "dongas," as they are locally
called, which sometimes present more striking and picturesque
appearances.

The apparent depth to which weathering has gone on is
often greater among siliceous than calcareous rocks. This is,
however, due merely to the facts that (1) the siliceous rocks
are composed largely of insoluble materials, and hence leave a
proportionately large amount of debris, and (2) that among
calcareous rocks the change is mainly chemical and takes place
only from the immediate surface. As a result of this, residuary
nodules of limestone may be found perfectly fresh and unal-
tered at a depth of but a few millimetres below the surface,
while granites and allied rocks may show signs of disintegra-
tion and incipient decay for many inches, or even feet.

Pumpelly states 4 that in the Ozark Mountains of Missouri
the secular dissolving away of limestones containing from 2 to
9% of insoluble matter has left residual clays from 20 to 120
feet in thickness, indicating a removal of not less than 1200
vertical feet by solution. According to Whitney, the dark,
reddish brown, residual clays of southern Wisconsin, of an
average depth of perhaps 10 feet over the entire area, repre-
sent the insoluble accumulations from the decomposition of
from 350 to 400 vertical feet of dolomite, limestone and calcareous
shale. As a considerable portion of the residue in any area un-
dergoing decay is being continually removed through the action

1 The Naturalist in Nicaragua, p. 86.

2 Am. Jour, of Science, February, 1884, p. 138.

3 Trans. Am. Inst. of Mining Engineers, Vol. XVIII, 1890, p. 337.

4 Am. Jour, of Science, 1879, p. 136.

KELATIVE AMOUNT OF MATEEIAL LOST 273

of running water, these figures, though suggestive and instructive,
fall far short of showing the full extent of the decay.

(10) Relative Amount of Material Lost. Other things being
equal, it is also safe to infer that more material has actually
been lost through disintegration and decomposition in moun-
tainous and hilly countries than from the level plains. This
for the reasons that (1) through the upturning of the beds there
were exposed, it may be, friable and soluble strata that might
otherwise have been protected, and (2) that through the shat-
tering incident to this upturning the rocks were rendered more
susceptible to the weathering forces. 1 Further, (3) the steeper
slopes in mountain regions promote more rapid removal of the
resultant debris, whereby fresh surfaces are continually exposed,
such as might otherwise shortly become protected through its
accumulation, as above noted.

1 According to Van Hise (Treatise on Metamorphism) minerals in a
condition of strain as commonly existing in compressed, folded and sheared
rocks, are more readily acted upon by underground solutions than when in
their normal condition.

PART IV

THE TRANSPORTATION AND REDEPOSITION OF
ROCK DEBRIS

IT rarely happens that more than a comparatively small pro-
portion of the products of disintegration and decomposition are
left to accumulate on the site of the parent rock. In most in-
stances a very considerable proportion, in some instances all, of
the debris is removed immediately, or soon after its formation,
and deposited elsewhere. A portion of this material is removed
in solution, as has already been described. A still larger portion
is transported mechanically, and it is to a discussion of the
method of this transportation that a few pages may now be
devoted with profit.

The chief agencies involved in this transportation are grav-
ity, water, in either a solid or liquid form, and the wind. Un-
doubtedly the major part of the work is done by water, but as
the wind's action is so frequently overlooked, and as, moreover,
the results thus produced are of more than ordinary interest
from the present standpoint, it may perhaps be well to dwell
upon this branch of the subject with considerable detail.

(1) Action of Gravity. Gravity, especially when aided by
the lifting power of frost, may locally exert no insignificant
influence. The tremendous power of landslides, or avalanches,
has, owing to their devastating effects, been impressed upon
us from the beginnings of written history. There are, how-
ever, other results, due to similar causes, but which, operating
on an almost microscopic scale, are wholly overlooked by the
ordinary observer, and the full meaning of which can be dis-
covered only when the results of years are taken into account.
Professor W. C. Kerr, in 1881, described 1 the manner in which
the superficial cap of soil from the decomposition of micaceous
and hornblendic gneisses near Philadelphia had crept down
the inclined surface on which it rested, and the gradual attenu-
ation of the bands of variously colored debris of which it was

1 Am. Jour, of Science, 3d Series, Vol. XXI, p. 345.

274

ACTION OF WATER AND ICE 275

composed. This creeping process he ascribed wholly to the
expansive action of included water passing into the condition
of ice, the expansion taking place laterally and the material
being pushed down the slope along the line of least resistance.
Mr. C. Davidson has since taken up the subject experimentally

FIG. 24. Showing direction and rate of motion of soil; the arrows showing,
by their relative lengths, the rate of movement at various points, a, soil;
6, bedrock.

and shown that the amount of the creeping could be accounted
for by the ordinary laws of gravity, the frost, by its expansion,
raising the individual particles a slight distance, and, on thaw-
ing, allowing them to drop back again a greater or less distance
down the slope, according to the angle of inclination. Dr.
Milton Whitney has, however, shown 1 that there is an almost
continual movement among soil particles, dependent upon
meteorological conditions quite aside from those involved in
freezing and thawing. The creeping appears therefore to be
but the manifestation, in mass, of the inclination of each indi-
vidual particle to slide down the slope.

The accumulations of talus at the foot of every cliff and on
the slopes of hills and mountains are matters of such every-day
observation as to need no mention in detail.

(2) The Action of Water and Ice. 2 The power of a stream
to transport rock debris depends naturally upon its volume
and the rapidity of its current. This, on the supposition that
the character of the sediment to be transported remains the
same. According to the calculations of Hopkins, as quoted by

1 Some Physical Properties of Soils, Bull. No. 4, TJ. S. Weather Bureau,
1892.

2 Students are referred to Professor R. D. Salisbury 's article on Agencies
which Transport Material on the Earth's Surface, Journal of Geology, Vol.
Ill, 1895, p. 70.

276 TEANSPOKTATION OF EOCK DEBRIS

Geikie, 1 the capacity of transport increases as the sixth power
of the velocity of the current; that is to say, the motor power
is increased sixty-four times, by doubling the velocity. The
following table is taken from the work quoted as showing the
power of transport of river currents of varying velocities :

INCHES MILES

PER SEC. PER HR.

3 0.170 : will just move fine clay.

6 0.240 : will lift fine sand.

8 0.4545: will lift sand as coarse as linseed.

12 0.6819: will sweep along fine gravel.

24 1.3638: will roll along rounded pebbles 1 inch in diameter.

36 2.045 : will sweep along slippery, angular stones of the size
of an egg.

There are, of course, other factors that should be taken into
consideration, such as the character of a river bed, the density
of the water, etc., but which lack of space prevents our touch-
ing upon here, and which are, moreover, sufficiently enlarged
upon in other works.

The writer has stood at the head waters of the Missouri, and
seen the Jefferson, Madison, and Gallatin rivers uniting their
floods to form one grand rushing stream of clear green water,
full of trout and grayling. He has seen it again at Mandan,
Dakota, a sluggish stream actually yellow with suspended silt.
At St. Louis it forms a mighty torrent, whirling along trunks
and stumps of trees, twigs, and all manner of organic debris
and inorganic detritus picked up from its banks, or washed in
by rains and tributary streams, till, one vast sea of liquid mud,
it pours every year into the Gulf of Mexico a mass of sediment
equal to 812,500,000,000,000 pounds (7,468,694,400 cubic feet),
or enough to cover a square mile of territory to a depth of 268
feet. But only a portion of the detritus carried by running
streams reaches the ocean; otherwise little attention need here
be given to its consideration. Nearly all streams, in some part
of their courses, flow through level plains with low banks which
are subject to inundation during seasons of high water. Con-
sider, then, a muddy stream such as is shown in cross-section
in Fig. 25, and which at ordinary periods is confined within
the. narrow channel near the centre. In time of freshet, however,
the volume of water is so greatly augmented as to cause it to

1 Text-book of Geology, 3d ed.

ACTION OF WATER AND ICE 277

overflow the banks and spread out over the plains on either hand.
But no sooner does the water leave the channel than the force
of the currents becomes checked, its carrying power lessened,
and it therefore begins to deposit its load of silt upon this flood
plain, as it is called, where it remains to permanently enrich the
land when the waters subside. It is to such processes of forma-
tion that are due some of the most fertile lands in existence, as
the valley of the Mississippi, that of the Bed River of the North,
the Nile, and scores of others that might be mentioned readily
attest. 1

To the same processes, coupled with the accumulation of
organic matter, we owe the filling in and gradual extinction of
thousands of glacial lakes throughout New England and the
North, and the formation of rich, flat-bottomed valleys known
locally as meadows, swales, and bogs.

Ice in the form of glaciers is an efficient agent for transpor-

FIG. 25.

tation as well as for erosion, as already noted. While the work
being done by existing glaciers may seem comparatively insig-
nificant, that done by the ice sheet of the glacial epoch was by
no means so, and deserves a more than passing notice. The
manner in which the ice carries and deposits its load has already
received attention in speaking of its erosive power, and but
little more need be said on the subject. That material which
existed in a loose, unconsolidated condition, on the surfaces on
which the glacier formed, was pushed and dragged along by

1 The Arkansas Kiver is stated by Owen (Geol. of Arkansas, 2d Eep., 1860,
p. 52) to be at certain seasons of the year almost blood-red from the quan-
tity of suspended fine ferruginous clay and saliferous silt brought down
from the regions of ferruginous shales, which prevail in the Cherokee County,
through which the river flows. This material, deposited along the banks and
in the eddies of still water, produces the celebrated red buckshot land.
Material washed from the bluffs of argillaceous shell marl, near the con-
fines of Jefferson and Pulaski counties, is deposited again farther down
the stream as a fine silt, imparting, like the red silt, extraordinary fertilizing
properties to the soil.

278 TKANSPOETATION OF KOCK DEBETS

the onward movement of the ice, which in extreme cases may
have exerted a pressure of 200,000 pounds to the square foot.
On the final retreat of the glacier, this was left in the form of a
compact structureless mass of almost stony hardness, commonly
known as till or ground moraine. Materials falling upon the
surface from greater heights were likewise transported, so long
as the ice sheet continued to advance, and finally deposited in
the form of terminal or frontal, medial and lateral moraines.

Inasmuch as the ice sheet was almost continually melting
upon its surface, it is practically impossible to consider its
action wholly independent of that of water also. Thus,
streams resulting from such melting would gradually wear
channels in the ice, as on the land. In these channels would
accumulate sand and boulders of such size and weight as to
resist the current, and such accumulations, on the final melting
of the sheet, would be deposited on the surface of the ground
in the form of ridges known as eskers, or osars. Other forms
produced by water action on the materials of the ice sheet, are
hillocks of stratified sand and gravel deposited near the terminal
moraines, and known as kames. Since during the advancing of
the ice sheet existing rivers flowing eastward must have been
dammed, we can safely imagine the formation of large tempo-
rary lakes, on the bottom of which would be deposited the
glacial silt, like the so-called loess of the Mississippi valley.
Lake Agassiz, a glacial lake of this type, is supposed to have
occupied an area of more than 100,000 square miles in north-
western Minnesota, northeastern Dakota, and a considerable
portion of Manitoba. On the bottom of this lake there was
deposited during the comparatively brief time of its existence,
silt to a depth as yet undetermined, but known to be at least
100 feet. 1

Waters issuing from the melting ice sheet tend to reassert the
material of the terminal moraine, redepositing it in approxi-
mately concentric zones beyond its margin. These deposits
are naturally thicker and coarser near the moraine and thinner
and finer at increasing distances. Their form and mode of
occurrence is such as to have suggested for them the name of
glacio-fluvial aprons, or frontal aprons. Their materials are
nearly always loose sands and gravels, the lithological nature

1 Ice Age in North America, by G. F. Wright, p. 355.

ACTION OF WATER AND ICE 279

of the individual particles being of course dependent upon that
of the moraines from which they are derived.

The effects upon the landscapes of this ice sheet have been
lasting and peculiar. One may safely imagine that, before its
invasion, the surface was covered with decayed and softened
materials like the residual soils of the Southern states, which
had been cut up into valleys and intervening ridges by the
stream of that time. The ice stripped from these surfaces
their mantle of decomposed materials, and in addition cut into
the fresh rock, actually planing the entire country so deeply that
the preglacial surface is no longer recognizable. The hills were
thus lowered and the valleys deepened or again filled by sand
and gravel. On its final retreat the surface, in many instances,
was left so thickly strewn with boulders that cultivation was
well nigh impossible prior to their renewal. The stone walls of
the New England farms were built not more for barriers against
roving cattle than to rid the fields of their material. (See
Fig. 2, PL 28.)

The direction taken by this drift material was quite variable.
It was, as a rule, from the north toward the south, with many
minor deflections. Boulders of Laurentian rocks north of Lake
Huron are abundant in the drift about Oberlin, Ohio, and even
further south. Boulders of native copper from the Lake Su-
perior region are found even as far south as Kankakee, Illinois,
and a large boulder of a peculiar conglomerate known in place
only near Ontario, has been found a few miles south of the
Ohio River in Kentucky. Dawson states "that boulders from
the Laurentian axis of the continent, which stretches from
Lake Superior northward to the west of Hudson Bay, have
been transported westward a distance of 700 miles, and left
upon the flanks of the Rocky Mountains at an elevation of
something over 4000 feet." 1

All over the states once occupied by this ice sheet the ma-
terial originating from the decomposition of rocks in situ, or
deposited on alluvial plains, was, with a few minor exceptions,
carried away to the southward and in part dumped into the
Atlantic, while its place was supplied by mongrel hordes from
the north. In process of digging for the foundations of the
Experiment Station at Orono, Maine, the fresh and highly
polished slaty rock was found but a few feet below the sur-

1 Ice Age in North America, p. 171.

280 TRANSPORTATION OF ROCK DEBRIS

face, proving incontestably that, with the exception of the
small amount of organic matter that had since been added,
not an ounce of the soil was truly native, but all of foreign
birth, and a mongrel creature of compulsory migration. We
shall dwell more fully upon the character and distribution of
these soils later. The single illustration above given will
answer present purposes.

In a less degree the ice along the shores of lakes and rivers
may exert a transporting influence. Thus the ice first formed
along the shores encloses sundry pebbles, boulders, and sand.
Through the expansive force of the freezing water as the entire
surface becomes frozen over, this shore ice, together with its
enclosures, may be pushed up some distance beyond the water
line, where the included debris is deposited on melting. Or,
on the breaking up of the ice in the spring, the shore ice may
be drifted to other parts of the lake, or down the stream, per-
haps for miles before melting sufficiently to cause it to deposit
its load.

(3) Action of Wind. 1 While abrasion by the wind is im-
possible without transportation, the converse is by no means
true ; indeed it is as an agent of transportation for rock detritus,
without appreciable abrasion, that the wind accomplishes its
greatest work, though in like manner this phase is most manifest
in arid regions.

It is stated by Darwin that for several months of the year
large quantities of dust are blown from the northwestern shores
of Africa into the Atlantic over a space some 1600 miles in
width and for a distance of from 300 to 600 and even 1000
miles from the coast. During a stay of three weeks at St. Jago
in the Cape Verde Archipelago, this authority found the atmos-
phere almost always hazy from the extremely fine dust coming
from Africa and falling upon the land and water for miles
around. So abundant was this dust that a distance of between
300 and 400 miles from the coast the water was distinctly colored

1 See article on Erosion performed by the Wind, by Professor J. A. Udden
Journal of Geology, Vol. II, 1894, p. 318. Attention is here called to the
fact that the speed of the wind upon which its power of transportation
depends is lowest near the ground, and hence that materials to be trans-
ported any great distance, at any one time, must be lifted through this
zone of low velocity. Professor Udden estimates that to be subject to trans-
portation by ordinary strong winds mineral particles must be comminuted
to not above one millimeter in diameter.

PLATE 26

FIG. 1. Forest destroyed by wind-blown sand.

FIG. 2. Wind drift and wind erosion. White Valley, Western Utah. U. S. G.

ACTION OF WIND 281

by it. In the arid lands of Central Asia the air is also reported
as often laden with fine detritus which drifts like snow around
conspicuous objects and tends to bury them in a dust drift
Even when there is no apparent wind, the air is described as
often thick with fine dust, and a yellow sediment covers every-
thing. In Khotan this dust sometimes so obscures the sun that
even at midday one cannot see to read fine print without the
aid of a lamp. The tales of the overwhelming of travelers and
entire caravans by sand storms in the Great Desert of Sahara
are familiar to every schoolboy. Greatly exaggerated though
these may be, the accounts of Layard and of Loftus show us that
the sand storms which are of frequent occurrence during the
early part of summer throughout Mesopotamia, Babylonia, and
Susiana are by no means of insignificant proportions. Layard
states that during the progress of the excavations at Nimrud,
whirlwinds of short duration but almost inconceivable violence
would suddenly arise and sweep across the face of the country,
carrying along with them clouds of dust and sand. Almost utter
darkness prevailed during their passage, and nothing could resist
their force ; the Arabs would cease their work and crouch in the
trenches almost suffocated and blinded by the dense cloud of
fine dust and sand which nothing could exclude.

The accounts of Loftus are equally impressive. Describing
their departure from Warka to Sinkara, he says: "A furious
squall arose from the southeast and completely enveloped us
in a tornado of sand, rendering it impossible to see within a
few paces. Tellig and his camels were as invisible as though
they were miles distant. A continuous stream of the finest sand
drove directly in our faces, filling the eyes, ears, nose, and mouth
with its penetrating particles, drying up the moisture of the
tongue, and choking the action of the lungs." With such
descriptions before one it is not difficult to believe that these
ruined cities have in the course of centuries been completely
hidden and their sites obscured by mounds of wind-drifted
sand and dust.

We need not, however, confine ourselves wholly to the Old
World for illustrations. Not longer ago than May of 1889 a
dry southwesterly wind which for several days had prevailed
in various parts of the Northwest, particularly in Dakota, cul-
minated in a storm peculiarly suggestive from a gelogical

282 TEANSPOETATION OF EOCK DEBRIS

standpoint. It is stated 1 that during the prevalence of this
wind, on the 6th and 7th of the month mentioned, the air be-
came filled with flying particles caught up from the ploughed
fields, fire-blackened prairies, public roads, and sandy plains.
The particles formed dense clouds and rendered it as impos-
sible to withstand the blast as it is to resist the blizzard
which carries snow in winter over the same region. The soil
to a depth of 4 or 5 inches in some places was torn up and
scattered in 'all directions. Drifts of sand were formed in
favorable places, several feet deep, packed precisely as snow-
drifts are packed by a blizzard. It seemed as if there were
great sheets of dust and dirt blown recklessly in mid air, and
when the wind died down for a few moments, the dirt, fine
and white, appeared to lie in layers in the atmosphere, clouding
the sun and hiding it entirely from sight for an hour or more
at a time. (See also on p. 163.)

Over the wide, dry, and bare flat-topped terraces of the upper
Madison valley the wind sweeps in a strong steady current
for days together, or during the heated portion of the year,
when the sun pours from a cloudless sky its hottest rays upon
the parched soil, starts up spasmodically here and there in the
form of small whirlwinds made visible by the dust they carry,
and which wander spectre-like across the plain to noiselessly
disappear in the distant mid air.

Dust columns of this nature are common in all arid regions,
and doubtless have been observed by the many who have
crossed the Humboldt desert in Nevada. Seated comfortably
in a Pullman car, one may at times see at a single view not less
than a half dozen of these geological spectres, each in the distance
doing its apportioned task and silently disappearing, laying down
its load of sand as its strength gives out and leaving it for its
successor. 2

Under proper conditions such of these wind-blown sands as
are too heavy to be carried into the air as dust accumulate
upon the surface in the form of drifts, or dunes, all lying with
their longer axes approximately at right angles with the pre-
vailing currents. Excepting during periods of calm, such are

1 American Geologist, June, 1889, p. 398.

1 Professor J. A. Udden estimates that the dust in a cubic mile of lower
air during a dry storm weighs not less than 225 tons, while in severe storms
it may reach 126,000 tons (Popular Science Monthly, September, 1886).

ACTION OF WIND 283

in a state of almost constant, though it may be imperceptible,
motion, ever changing their shapes and moving onward like
long parallel drifts of snow. The rate of motion of a dune
from necessity is governed by the strength and constancy of
the winds, and the fineness and dryness of the sand. Urged
into temporary activity, each little grain goes scurrying up the
slope, across the crest, and tumbles to rest in the steeper
declivity upon the leeward side, to be slowly buried by those
which follow. This is the sum total of the movement taking
place in the march of a dune, whatever its pace and however
great its bulk. Yet in this very faculty of moving itself for-
ward by but a ten billionth part of its bulk at a time lies the
whole secret of its power. Silently, imperceptibly it may be
except when measured by months and perhaps years of time,
retarded by no walls nor ordinary declivities, it relentlessly
performs its task. 1

A writer in one of the recent popular magazines estimates
the dunes of Hatteras and Henlopen as in some cases upwards
of 70 feet in height and moving at least 50 feet a year. Swamps
have thus been filled, forests and houses buried, and it is stated
that but a few years can elapse before the entire island lying
north of Cape Hatteras will be rendered uninhabitable. The
sand dunes on the coast of Prussia commenced but little more
than a century ago, and already fields and villages have been
buried and valuable forests laid waste by them. In one instance
a tall pine forest covering many hundred acres was destroyed
during the brief period intervening between 1804 and 1827.
Loftus, writing of Niliyga, an old Arab town a few miles east
of the ruins of Babylon, says that in 1848 the sand began to
accumulate about it, and in six years the desert within a radius
of six miles was covered with little undulating domes, while
the ruins of the city were so buried that it is now impossible
to trace their original form and extent. A still more striking
illustration of the rapidity of sand accumulations is offered
by the same authority in describing the burial customs of some
of these ancient people, it being stated that the earthen coffins
were merely stacked in layers one on top of another, and left
thus to be covered by the finer sand sifted over them by the
winds from the desert. Even Nineveh, founded some twenty
centuries before Christ and destroyed 1400 years later, became

1 The Wind as a Factor in Geology, Engineering Magazine, 1892, p. 596.

284 TEANSPOETATION OF EOCK DEBRIS

so covered by drifted sands that at the time of the Greek
Xenophon (about 400 B. c.) the very site of the once famous
city was unknown. Marsh 1 gives the rate of movement of dunes
along the western coast of Jutland and Schleswig-Holstein as
averaging 13 J feet a year, while Anderson estimates the aver-
age depth of the sand over the entire area as about 30 feet,
equalling therefore about 1J cubic miles for the total quantity.

It is not in all cases possible to trace the drifted sands to
their various sources. Dunes along the sea-coasts are in nearly
all cases composed of materials thrown up by the waves on
the beaches in the immediate vicinity. This is the case with
those of Hatteras, Cape Cod, Gascony, Algeria, and Schleswig-
Holstein. But the origin of the large inland dunes, like those
of Nevada, is not always so clear. It has been suggested that
these last are formed of beach sand driven in by the prevail-
ing westerly winds from the Pacific coast. This is, however,
a matter of very grave doubt, and it seems more probable, as
stated by geologist Russell, 2 that they were derived from the
disintegrating granites of the Sierras. They certainly have
traveled far, and are not a product of disintegration of rocks
in the immediate vicinity. 3

By wind action, accompanied by the carrying power of spas-
modic or perennial streams, were formed the wide stretches
of adobe in the western United States, and according to many
authorities the deposits of loess which cover, as in Europe and
Asia, areas aggregating many square miles and have a depth,
in extreme cases, of 2000 feet. 4

'The Earth as Modified by Human Action, p. 562.

2 Quaternary History of Lake Lahonton, Nevada, Monograph, U. S.
Geol. Survey, 1885.

8 The sands covering the Egyptian Sphinx and Pyramids are stated to
have come mainly from the sea on the north, and not from the desert, as
is popularly supposed. Sand showers having their origin in the desert of
Sahara extend across the Mediterranean, and as far as northern Italy
(Nature, July 18, 1889, p. 286).

*The wind plays an important part in the transportation of soils in
Wyoming, owing to their incoherent state, which is due to a lack of clay.
The arid regions of this state, which are chiefly Tertiary and Cretaceous
plains and tablelands, receive very little rain. Consequently the soils be-
come loosened, and during the dry and windy winter weather are trans-
ported to the broken country and distant hills and mountains in dense
clouds, which almost suffocate travellers. In a single season it is not an
uncommon sight to see banks of earth, like huge banks of snow, behind a
reef of rock, or in the lee of large bunches of sage brushes (U. S. Dept. of
Agriculture, Office of Experiment Stations, Vol. V, No. 6, 1894, p. 567).

ACTION OF WIND 285

The tendency of the wind is not, however, in all cases toward
forming drifts and ridges, but at times rather to reduce the
land to one general level. J. Flinders Petrie 1 states that near
the ancient cemetery of Tell Nebesheh, on the Isthmus of Suez,
the surface of the country has been cut down at the rate of 4
inches a century until some 8 feet have been removed from
the dry areas and deposited in the intervening depressions,
slowly converting the existing lakes into marshes, and the
marshes into dry land. An even more rapid change of con-
tours is that described by Dwight 2 as having taken place on
Cape Cod, Massachusetts. The entire country here is com-
posed of sand so susceptible to the drifting action of the wind
that it has for years been the custom of the people to sow pines
and coarse beach grass to hold it in place. In the instance
described by Dwight, however, reckless pasturage had so far
destroyed the grass as to lessen its protecting power, and
under the strong breezes from the open Atlantic it began to
drift rapidly. Over an area of about 1000 acres the sand was
blown away to a depth, in many places, of 10 feet. "Nothing,"
says Dwight, "could exceed the dreariness of this scene. Not a
living creature was visible; not a house, nor even a green thing
except the whortleberries which tufted a few lonely hillocks
rising to the height of the original surface, and prevented by
this defence from being blown away also. The impression made
by this landscape cannot be realized without experience. It
was a compound of wildness, gloom, and solitude. I felt
myself transported to the borders of Nubia, and was well
prepared to meet the sand columns so forcibly described by
Bruce, and after him by Darwin. A troup of Bedouins would
have finished the picture, banished every thought of my own
country, and set us down in an African waste."

One more instance of contour changes of this sort must suffice.
It is stated 3 that in Pipestone and Rock counties in Minnesota,
the bluffs facing to the westward are, as a rule, more precipi-
tous and more rocky than those facing in the opposite direction.
This is regarded by Professor Winchell as due to the action of
the prevailing westerly winds, combined with the drying effects
of the southwestern sun in summer. The winds uncover and

. Koyal Geographic Soc., November, 1889, p. 648.
2 Travels in New England and New York, Vol. Ill, p. 101.
8 Geol. of Minnesota, Vol. I, p. 575.

286 TKANSPOKTATION OF KOCK DEBBIE

keep bare the coarse materials of the western surface by blowing
away the sand and clay, while the protected bluffs on the east
collect upon their slopes all the flying particles from the prairies
above.

The finely comminuted rock dust blown from volcanic vents
is often drifted for long distances by atmospheric currents, and
ultimately deposited in beds of no insignificant proportions.
Dense clouds of such dust were blown from Icelandic volcanoes
to the coast of Norway in 1875, and subsequent to the eruption
of Krakatoa (in 1883) the ship Beaconsfield of Philadelphia,
while at a distance of 831 miles from the source, sailed for three
days through clouds of dust which fell upon her decks at the
rate of an inch an hour. That such are not or have not in
the past been unusual instances is shown by results obtained
by the Challenger Expedition, volcanic ashes and sand being
repeatedly dredged up from almost abysmal depths at points
in the central Pacific far remote from land areas. The day
following the explosive eruption of St. Vincent, in 1812, the
Barbadoes Island, 80 miles to the windward, was completely
shrouded in darkness for many hours, the light of the sun being
almost wholly obscured by the cloud of impalpable dust which
in the form of a slow, silent rain fell over the whole island.
1 'The trade wind had fallen dead; the everlasting roar of the
surf was gone; and the only noise was the crushing of the
branches snapped by the weight of the clammy dust. About
one o'clock the veil began to lift, a lurid sunlight stared in
from the horizon, but all was black overhead. Gradually the
dust cloud drifted away; the island saw the sun once more,
and saw itself inches deep in black, and in this case fertiliz-
ing dust." 1 The late eruptions of St. Vincent and Martinique
(1902), as described by numerous writers, furnish still more im-
pressive illustrations of the enormous amount of detrital material
ejected during a single period of eruption, and of its wide dis-
tribution.

'Kingsley, as quoted by Belt, in The Naturalist in Nicaragua, p. 354.

PART V

THE REGOLITH

THROUGHOUT all the the millions of years which have elapsed
since the earth assumed its present form and essentially solid
condition, the rocks composing its more superficial portions have
been constantly undergoing degeneration in the manner de-
scribed, and, in so doing, have given rise to the immense masses
of materials which constitute the thousands of feet of secon-
dary rocks and the still unconsolidated sands, gravels, and other
products which will be considered in detail later. With those
products which have undergone lithification, which are now in
the state of consolidation commonly ascribed to rocks by the
popular mind, we shall have little more to do. These have
already been sufficiently described as rocks in Part II of this
work. It is to the most superficial and unconsolidated portion
of the earth's crust that we will now devote our attention.

Let the reader for a moment picture to himself the present
condition of this crust, with particular reference to the land
areas. Everywhere, with the exception of the comparatively
limited portions laid bare by ice or stream erosion, or on the
steepest mountain slopes, the underlying rocks are covered by
an incoherent mass of varying thickness composed of materials
essentially the same as those which make up the rocks them-
selves, but in greatly varying conditions of mechanical aggrega-
tion and chemical combination.

In places this covering is made up of material originating
through rock-weathering or plant growth in situ. In other
instances it is of fragmental and more or less decomposed mat-
ter drifted by wind, water, or ice from other sources. This
entire mantle of unconsolidated material, whatever its nature
or origin, it is proposed to call the regolith, from the Greek
words /fyf7, meaning a blanket, and \iOo^ y a stone* Within
certain limits it varies widely in composition and physical proper-

1 From a strict philological standpoint the word, it will be noted, should
have been spelled rJiegolith.

287

288

THE EEGOLITH

Transported j

ties, and many names have, on one ground and another, been ap-
plied to its local phases, the more important of which are given in
tabular form below, and described in detail in the pages following.
According to its origin, whether the product of transporting
agencies as noted above, or derived from the degeneration of
rocks in situ, the regolith is found lying upon a rocky floor of
little changed material, or becomes less and less decomposed
from the surface downward until it passes by imperceptible
gradations into solid rock.

SUBDIVISIONS OF THE EEGOLITH

r Residuary gravels, sands and
f Residual deposits -I clays, wacke, laterite, terra
Sedentary J ( rossa, etc.

Cumulose deposits J Pcat > muck > and swamp soils, in
L \ part.

n . , T .. f Talus and cliff debris, material
Colluvial depositsj of avalanches .

The I . f Modern alluvium, marsh and

regolith-j AlluvialJepositsJ swamp (paludal ') deposits, the

Champlain clays, loess, and

adobe, in part.
Wind-blown material, sand

dunes, adobe and loess, in part.
Morainal material, drumlins, es-

kers, osars, etc.

The extreme upper, most superficial portion of this regolith,
that which affords food and foothold for plant life, is commonly
designated as soil; that immediately underlying the soil, and
passing into it by insensible gradations, is known as the sub-soil.
This last differs from the soil proper only in degree of compact-
ness and in such chemical changes as may have been induced
in the soil through growing organisms and more extensive
weathering. Indeed, the soil is but derived from the sub-soil,
and were it entirely removed, would shortly be replaced through
the same agencies as first gave it birth.

The characteristics of individual soils can best be discussed
when speaking of those local phases of the regolith of which
they form a part, and with this understanding we will proceed.

1. SEDENTARY MATERIALS

Here are to be considered those deposits which, resulting
from chemical decomposition or disintegration, from any or all
of the processes involved in rock-weathering, or from organic
accumulation, are found to-day occupying their original sites.
They are, in fact, the primeval types of nearly all soils and sec-

Alluvial deposits
(including aqueo-
glacial)

JEolian deposits
Glacial

SEDENTARY MATERIALS: RESIDUARY DEPOSITS 289

ondary rocks, since those of drift origin are but derived from
sedentary materials through the transporting agencies of air
and water. They may be
conveniently divided into
two classes, (1) residual 1
and (2) cumulose.

(1) Residuary Deposits.
Under this name, then,
are included all those
products of rock degenera-
tion which are to-day
found occupying the sites
of the rock masses from
which they were derived,
and immediately overlying
such portions as have as
yet escaped destruction.
The name is peculiarly ap- FIG. 26. Showing angular outlines of re-
propriate, since they are siduary particles from decomposed gneiss,
actually residues, left be- X ' mica ' 2 > felds P ar J 3 > 1 uartz -
hind while the more soluble portions have been leached away by
meteoric waters.

The residual deposits of North America reach their maximum
development in the portion of the United States east of the
Mississippi and south of the southern margin of the ice sheet
of the Glacial epoch. Their mode of accumulation and general
characteristics have been very thoroughly discussed by Professors
Russell, Chamberlin, and Salisbury, 2 on whose papers we shall
draw for some of the facts given here.

1 Various names have from time to time been proposed for deposits of this
nature, but obviously it is impossible to include under a single lithological
term materials so widely variable. The term saprolite (from the Greek
o-dTTpps, rotten, recently suggested by G. F. Becker, 16th Ann. Rep. U. S.
Geol. Survey, Part III, p. 289) is objectionable as conveying the idea of
putridity. Moreover although resulting from the rotting of rocks the soil
cannot, in itself be considered as rotten. The old provincial term geest
adopted by De Luc, and recently endorsed by McGee (llth Ann. Rep. U.
S. Geol. Survey, 1889-90, p. 279), has lost whatever precise meaning it may
have had, being defined in both the Standard and Century dictionaries as
(1) a bed derived from rock decay in situ, (2) high gravelly land, and (3)
gravel or drift. The term gruss, although advocated by some American
authorities, is of old German origin and open to the same objection.

2 Bull. 52, U. S. Geol. Survey and Ann. Rep. U. S. Geol. Survey, 1884-85.
20

290 THE KEGOLITH

The prevailing characteristic of an old residual deposit, from
whatever rock it may be derived, is a ferruginous clay. Exam-
ined by a microscope, its mineral particles, when not too thor-
oughly decomposed, are found to be sharply angular in outline.
With the exception of the quartz, the various mineral constitu-
ents are often in an advanced stage of decay, and the more
soluble constituents are wholly or partially lacking, having been
leached out, in the manner already described. (See under Soil,
p. 345.)

The colors are dull, or some shade of brown or red, owing to
the higher oxidation and perhaps dehydration of the ferruginous
matter set free by the decomposition of the iron-bearing sili-
cates. Such in general are the residual soils of the southern
Appalachian regions of the United States which are apparently
comparable with the terra rossa of Europe, but only in a slight
degree with the laterite of India, to which they have often un-
fortunately been referred. 1 From a chemical standpoint the
soils forming the upper portion of the residuary deposits vary
widely from the rock masses from whence they were derived,
much depending upon their age and the amount of actual de-
composition and leaching that has taken place. On p. 347 are
given a few typical but widely varying analyses which will serve
to illustrate this point.

Deposits of this nature are never truly stratified, excepting
where, through having remained wholly undisturbed, they re-
tain the original structure of the parent rock. (See under
Effacement of Original Characteristics, p. 249.)

The residuary differ from the drift deposits in that they con-
tain no materials foreign to their vicinity, but only such more
enduring matter as has been handed down to them from the
parent rock. In the case of limestones such matter consists
mainly of aluminous and ferruginous matter, grains of sand,

1 The term terra rossa, according to Neumayer (Erdgeschichte, Vol. I, p.
405) was first applied to the red residual deposits in the Karst maritime
lands of the Adriatic Sea. The material is described as a highly ferruginous
clay resulting from the leaching out, by meteoric waters, of the soluble
portions of the prevailing limestones. Its distribution is by no means limited
to the maritime provinces of the Karst, but it is found also on the Grecian
coasts and in the Schwabia-Frankonia Jura Plateaus of Bavaria. In fact
it is to be found anywhere in regions where the prevailing country rock is
a marine limestone and erosion not sufficiently active to remove the residu-
ary material.

SEDENTARY MATERIALS: RESIDUARY DEPOSITS 291

and nodular masses of chert which existed as mechanically ad-
mixed impurities.

The inherited characteristics of deposits of this nature may
be illustrated by the accompanying exaggerated section across
central Kentucky where, it is easy to see, the regolithic mate-
rial overlying the Lower Silurian and Cambrian limestones may
contain a portion of all the insoluble residues from the hundreds

FIG. 27. Diagram showing the successive variations of fertility in the
soils of central Kentucky during the downward movement of the rocks.
a, a, a, parts of the present surface enriched by decay of limestones; b,
next preceding stage, when soils rested on Devonian shales and were moder-
ately fertile; c, yet earlier stage, when soils were formed on millstone
grit and were very lean; d, earliest stage when soils rested on the coal
measures, and were moderately fertile. For simplicity of illustration
several stages of variation are omitted. After N. S. Shaler.

of feet of Upper Silurian, Devonian, Lower and Upper Carbon-
iferous beds which formerly stretched above them. Upon the^-~
nature of this inheritance must depend the adaptability of the
regolith to soil purposes and its consequent fertility. 1

The transition from a regolith of this type to fresh rock is
usually quite sharp, owing to the fact that limestones decompose
mainly through solution from the immediate surface. Never-
theless there is a gradual change in the character of such a
deposit from above downwards, owing to the oxidizing influence
of the air and percolating waters. (See p. 243.)

As above noted, the mineral particles in the older residuary
deposits are, with the exception of the quartz, found to be as a
rule in a state of advanced decomposition. Nevertheless the
ultimate individual constituents of even the darkest clays of the
driftless regions of Wisconsin, as examined by Messrs. Chamber-
lin and Salisbury, are transparent, although stained by iron
oxides.

Concerning the physical properties of limestone residues as
occurring in this driftless area, the following statements are

1 The limestones of the Boone formation, near Talequah, Indian Territory,
contain so large an amount of chert nodules, as to render the residual soil
unfit for cultivation, and suitable only for the growth of forest trees. ( J. A.
Tafft, Folio 22, U. S. Geol. Survey.)

292

THE KEGOLITH

made by Messrs. Chamberlin and Salisbury: " Above, the
clay graduates into soil which, outside the valleys, is uniformly
shallow. Beneath the soil, the clay loses the dark color of the
latter, due to the presence of organic matter, but is for a certain
distance down ward -not unlike the superior portion in texture.
The deeper lying clay, where limestone is the subjacent rock,
is the most characteristic member of the residuary earth series.
It is not like that above, structureless, although, like that, it is
without trace of stratification. It generally shows a tendency to
cleave, breaking up into little pieces which are roughly cubical.
This is often conspicuous, and especially so on the faces of

sections which are thor-
oughly dry. In such situ-
ations large quantities
of the clay in small angu-
lar blocks may be removed
by slight friction. The
size of the cuboids varies,
within somewhat narrow
limits, from a small frac-
tion of an inch to one or
two inches in diameter.
This cleavage is probably
a phenomenon of shrink-
age due to drying, as it
partially disappears when
the clay becomes wet.
This structure has given
rise to the local name of 'joint' clay, an appellation not alto-
gether inappropriate.

"Upon drying, this variety becomes very hard and rock-like.
It only becomes adapted to serve as soil by surface amelioration,
as is shown by the fact that, from the thousands of mineral holes,
scattered over the southern part of the mining district, the
material ejected still lies beside the excavations as heaps of clay,
without covering of vegetation, although it has been exposed in
most cases for many years. Notwithstanding this fact, the
clay, even in its deepest parts, wherever examined, is found to
abound in minute perforations. These, in many cases at least,
indicate the penetration of rootlets, for the rootlets themselves.

FIG. 28. Showing angular character of
quartz particles in decomposed gneiss.

SEDENTARY MATERIALS: RESIDUARY DEPOSITS 293

may sometimes be found. In some cases, too, the perforations
have been seen to undergo a gradual variation in size, and to
branch now and them, much as rootlets do. On the other hand,
it is probable that some of the perforations have had a different
origin, for in one case a small insect was found in one of the
little canal-ways. The clay is exceedingly tenacious, and hence
the perforations, once formed, would endure for long periods of
time.

"Another characteristic of certain portions of the clay is its
power of retaining moisture. It can rarely be found, even in
the driest season, unless exposed to the direct rays of the sun,
without visible moisture a few inches from the surface. The
regions where it is present are conspicuously less affected by
drouth than adjacent localities where it is wanting. For this
reason it is a valuable sub-soil.

"Fragments of residuary rock are not uncommon in the deeper
portions of this earth. Of these, chert fragments are most
abundant, and occur scattered sparingly throughout the clay or
sometimes arranged in more or less distinct layers in it. Even
where they appear to be entirely wanting, the microscope often
reveals minute flakes scattered sparsely throughout the clay.
The larger pieces are more numerous near the basal portion of
the clay than higher up.

"It is natural to suppose that the residuary earths derived
from the decomposition of limestone would differ very notably
from those which take their origin from sandstones or from
shales or mixed crystalline rocks. Yet the difference is far
less than might be anticipated. There usually overlies the
sandstone strata a loamy earth not very far removed in char-
acter from that which mantles limestones. It is somewhat
more sandy, and consequently less cohesive, and presents the
opposite variations in vertical sections, becoming less cohesive
below, instead of more so. In the limestone region the toughest
clay lies next to the rock. In the sandstone regions the soil
graduates below into sand. The difference is most conspicuous
where the mantle has been washed and redeposited and mingled
with mechanically derived sand and secondary products, as
occurs in some of the valleys." 1

The following analyses, in part from this same report, will
answer, in connection with those already given, to show the

1 6th Ann. Rep. U. S. Geol. Survey, 1884-85, pp. 240-242.

294:

THE EEGOLITH

os eo

CO rH

S :

O5 CO

;

iO O
O (M

-w

If 5

: :

a s

o 8

;

co ;

52
S g
% %

d I

os
o

0) O>

2 I

H H

co I

CO
CO

CO
rH

OS
I-

CO 00

rH rH

CO
CO

O CO
<# rH

r <M
CM

^ rH
(H
CM

co'

;

rH CO

OS I

52
1

CO CO
rH

O OS

co co

rH O
rH

52
OS CO

co co

O CO

co co

3 S3

OS OS

r^ <M

CM
<N

CO rH

CO O

o o

rH O

o d

OS CO
OS CO

OS 1>-

CO
CNI

d d

co
d

CO Oi

d d

o d

co
d

I-H

CO CO
rH CM

CM >O

O Tfl

*O O

d d

OS rH
rH CO

CO CO

"* d

/-N /S

CONSTITUB)

Silica (Si0 2 ) . .
Alumina (Al 2 0a)

Ferric oxide (Fe 2 3
Ferrous oxide (FeO

Titanium oxide (Ti

Phosphoric acid (P 2
Manganese (MnO)

\_>

a
3

(M k>*

^"H

\^^/ M

CC PH

O
' O

O~ 'o

SEDENTARY MATERIALS: RESIDUARY DEPOSITS 295

prevailing type of the residuary deposits throughout widely
separated areas. It will be noted that silica as a rule exceeds
all other constituents, while alumina, iron oxides, and moisture
make up the main bulk of the residue. This generalization
holds good for nearly all sedentary soils, whatever the character
of the rocks from which they were derived, and is the more
pronounced the more advanced the decomposition.

Columns I, II, III, and IV of this table (see opposite page)
are limestone residuals from southern Wisconsin. Columns I
and II are from the same vertical section, I being 4J feet from
the surface, and II 8-J, and in contact with the underlying lime-
stone. Columns III and IV are similarly related, III being 3
feet from the surface, and IV 4J feet, the lower sample lying on
the unchanged rock. The larger percentages of silica in the
samples from nearest the surface indicate a higher state of
decomposition, the soluble constituents having been more com-
pletely removed. The presence of large percentages of alkalies
in these same samples indicates that these salts existed in the form
of silicates which have resisted the decomposing influences, and
remain mechanically included in the residues. Column V is a
clay from the decomposition of the Knox dolomite at Morris-
ville, Alabama ; VI the characteristic red earth from the decom-
position of coralline limestone on the islands of Bermuda; VII
a product of the decay of a diabase dike at Wadesboro, North
Carolina; VIII a gabbro sub-soil from Maryland; IX a sub-soil
from the decomposition of Trenton limestone near Hagerstown,
Maryland; and X a residual soil from the decomposition of a
Triassic sandstone, Maryland.

A microscopic examination of the material represented by
analyses I and IV, as given by the authorities quoted, showed
it to consist of particles in an extreme condition of comminution.
An actual measurement of over 700,000 of these particles yielded
results as below :

Particles less than .0025 mm. in diameter 721866

Particles between .0025 mm. and .005 mm. in diameter .... 9812
Particles over .005 mm. in diameter 0634

732312

Of those over .005 millimetre in diameter, particles reaching
0.06 millimetre were not rare. Nearly all those above 0.1 milli-
metre were found to be of flints and cherts which graded up

296

THE KEGOLITH

into chips and flakes of notable sizes. Particles much coarser
than those above enumerated occur, but their actual number is
comparatively small, though their comparative bulk may be
considerable.

Work of a like nature, but done under somewhat different
conditions, by Dr. Milton Whitney, showed the residues from
the Trenton limestones near Hagerstown, Maryland, to contain
on an average some 45% of finely comminuted material, the
individual particles of which vary in size between .005 and
.0001 millimetre in diameter, and which may appropriately
be termed clay. As Dr. Whitney has calculated, there are
approximately 22,000,000,000 grains of sand and clay in each
gramme of such a sub-soil, presenting in every cubic foot not
less than 158,000 square feet of surface to the action of water
and air, as well as to the roots of growing plants.

The results of mechanical analyses of (I and II) residues from
the Trenton limestone, (III) Triassic sandstone, (IV) gabbro,
and (V) gneiss are presented in tabular form below. 1

MECHANICAL ANALYSIS OF EESIDUAL DEPOSITS

DIAMETER

PARTICLES

MM.

CONVENTIONAL NAMES

III

2 1

Fine gravel ... . .

0.54

0.17

0.00

000

0.19

1-5

Coarse sand

0.32

0.00

0.23

1 50

1.80

.5-25

0.72

0.15

1.29

349

3.12

.25-.!

0.62

0.25

4.03

624

6.96

1-05

Very fine sand

4.03

2.34

11.57

11 74

8.76

.05-.01

Silt

36.02

19.04

38.97

3260

34.92

.01-.005

14.99

20.88

8.84

10.77

12.14

.005-.0001

Clay .

41.24

51.77

32.70

26 62

28.82

Total mineral matter ....
Organic matter, water, and loss

98.48
1.52

94.60
5.40

97.63
2.37

92.96
7.04

96.71
3.29

100.00

Many of the products of weathering of siliceous crystalline
and calcareous rocks are of economic importance as soils, clays,
and iron ores, as elsewhere noted. The kaolin beds of northern

1 Bull. No. 21, Maryland Agr. Exp. Station, 1893.

SEDENTARY MATERIALS: RESIDUARY DEPOSITS 297

Delaware and southwestern Pennsylvania are mainly decom-
posed, highly feldspathic, gneissic rocks, which as dug from the
pits still retain their gneissic structure, but are now plastic clays
full of angular quartz fragments, mica scales and feldspar par-
ticles in various stages of decomposition. The change that has
taken place consists in a kaolinization of the feldspars, whereby
the alkalies are largely removed, and a residue consisting essen-
tially of a hydrous silicate
of alumina left in their
place. The quartz gran-
ules are disaggregated,
and their surfaces some-
times slightly etched by
the action of the alkaline
carbonates; the black Y
mica, where such existed,
decomposed, giving rise
to rust-colored spots.
The material is dug from
the pits and washed with
water to separate the im-
purities, the "kaolin'* or
clay remaining in suspen-
sion, and being ultimately
saved by filtration through
canvas. This finest material, as seen under the microscope, still
contains particles of undecomposed feldspars and shreds of white
mica, together with other extremely irregularly outlined, some-
times almost amoeba-shaped forms, as shown in Fig. 29. An
average of two mechanical analyses of this clay, made under Dr.
Whitney 's direction, yielded the results given below :

^.-Showing, on the left, the mineral

kaolinite as seen under the microscope.
and on the right; washed kaolin<

MOISTURE IN AIR-
DRY MATERIAL AT
100 C.

MOIBTURE ON
IGNITION

SILT

.05-.01 MM.

FINE SILT
.01-.005 MM.

CLAY
.005-.0001 MM.

0.41 %

H-41%

31.79%

7.31%

47.78%

Chemical analyses of the same material, made in the laboratories
of the United States Geological Survey, yielded :

298 THE BEGOLITH

CHEMICAL ANALYSIS OF KAOLIN, HOCKESSIN, DELAWARE

Silica (SiO 2 ) 48.73%

Titanic oxide (TiO 2 ) 0.17

Alumina (A1 2 O 8 ) 37.02

Ferric iron (Fe 2 O 8 ) 0.79

Lime (CaO) 0.16

Magnesia (MgO) 0.11

Potash (KsO) 0.41

Soda (Na 2 0) 0.04

Water at 100 0.52

Ignition 12.83

Phosphoric acid (P 2 O 5 ) 0.03

Total 100.81%

Among the special names that have from time to time been
given to local phases of residuary accumulations, there remain
two, the laterite and Wacke, which are sufficiently common to
merit some attention. The first mentioned of these, laterite,
like loess and several other terms that might be mentioned,
has to a considerable extent lost its true lithological signifi-
cance through a careless usage. Originally the name was applied
to a vesicular highly ferruginous clay, soft in the mass, but hard-
ening on exposure to the weather, which has a wide distribu-
tion throughout India and Ceylon. Two forms are commonly
recognized, the one capping the summits of hills and plateaux
on the highlands of central and western India, and underlaid
by the Deccan traps ; and the second occurring on the lowlands,
in part overlying gneisses and granites. The prevailing colors
of the laterite, when freshly broken, are various tints of brown,
red and yellow mottled, or whitish ; after exposure it is usually
covered with a brown or blackish-brown coating of limonite.
When first dug out, the material is sufficiently soft to be cut
with a pick or shovel, but becomes greatly indurated on expo-
sure. In some instances the material is of so compact a texture
and so hard as to resemble jasper. In many forms of laterite
the material is traversed by "small irregular tortuous tubes
from a quarter of an inch to upwards of an inch in diameter."
These penetrate the mass in all directions, though most com-
monly nearly vertical, and are often lined with a coating of
limonite. On weathering, these give rise to extremely irregu-
larly pitted or scoriaceous surfaces, which, together with the
dense, often botryoidal structure, cause it to resemble certain
types of igneous rocks, for which it has more than once been

SEDENTARY MATEEIALS: RESIDUARY DEPOSITS 299

mistaken. The more massive forms show usually a horizontal
banding. Some forms of laterite show a brecciated structure,
due to its detrital fragments becoming recemented into masses
closely resembling the original rock. The high level form, that
which occurs capping the hills and plateaux on the highlands
of central and western India, is fine grained and compact and
of a fairly homogeneous structure, although the iron oxide may
be somewhat irregularly distributed and sometimes segregated
in pisolitic nodules sufficiently abundant to form an ore. The
lower level form, that which covers large areas of both east and
west coasts, frequently contains grains of sand and pebbles
embedded in a ferruginous matrix. It is, as a rule, less homo-
geneous than the high level form, but nevertheless passes into it
by insensible gradations.

The origin of both high and low level forms of the laterite
has been the subject of much speculation. It is probable that
all of it is of a residual nature, i. e., represents the less soluble
portions of pre-existing rock masses. That which is found on
the high levels occurs overlying the Deccan trap sheets, into
which it can in many instances be traced, proving conclusively
its origin from this rock by the ordinary processes of weather-
ing. The low-lying variety can, in many instances, in like man-
ner be traced back to its origin from more siliceous, gneissic, and
granitic rocks. A part of the material, however, has the ap-
pearance and structure of a clastic rock of sedimentary origin,
and so it is considered, by the best authorities, to be.

The chemical composition of a very ferruginous laterite is as
below :

CHEMICAL ANALYSES OP LATERITE, RACOON, INDIA

CONSTITUENTS

INSOLUBLE

SOLUBLE

BULK

Silica (Si0 2 )

30.728%
I 2.728 I

I 6.802

6.848%
6.783
46.279
0.742
0.090

37.576%
1 55.532

6.892

Alumina (AloOa) .

Iron sesquioxide (Fe2O3) .

Lime (CaO) . . ....

Magnesia (MgO) .

Alkalies

Water and loss

40.258

59.742

97.270

100.00%

300 THE KEGOLITH

"The surface of the country composed of the more solid
forms of laterite is usually very barren, the trees and shrubs
growing upon it being thinly scattered and of small size. This
infertility is due, in great part, to the rock being so porous that
all the water sinks into it, and sufficient moisture is not retained
to support vegetation. The result is that laterite plateaux are
usually bare of soil, and frequently almost bare of vegetation. ' ' 1

Wacke is an old German name now but little used, designating
the gray, brown to black earthy residue or clay resulting from
the decomposition in place of basic eruptive rocks, as basalt,
melaphyr, etc. In composition the material naturally varies
with the character of the rock from which it was derived, and
the amount of decomposition and leaching it may have undergone.

It seems advisable to call attention here, a little more emphatic-
ally, to the fact that the same processes which in ages past have
been instrumental in the formation of sandstones, shales, slates,
or marls are to-day, and have in late Tertiary and in Quaternary
times, given us soils; in other words, many of our soils are but
secondary rocks in a state of loose consolidation, and many of
the accumulations classed as residual were derived by disintegra-
tion, in situ, of alluvial materials ; materials brought down years
ago and deposited in shallow seas. The amount of consolidation
undergone by the more recent of these sediments has in many
instances been so slight that on elevation above the water level
they are ready almost at once to assume the role of soil with little
if any preparatory disintegration. Nevertheless consistency de-
mands that such be here grouped as residuary.

Over what is known as the coastal plain of the middle Atlan-
tic slope, a narrow belt bordering on the Atlantic and extending
from the Hudson River on the north to the Eoanoke on the
south, have been deposited in late Mesozoic and Tertiary times
a series of gravels, sands, and clays which constitute the well-
known Potomac, Appomattox, and Columbian formations of
Darton, McGee, and others. These are all detrital deposits
from the eastern Appalachian regions, brought down by streams
and deposited in the shallow estuaries and deltas of these

1 Manual of the Geology of India, by E. D. Oldham, 2d ed., 1893, pp.
369-390. Max Bauer has since shown (Neues Jahrb. fur Min., etc., 1898,
Vol. II, p. 163) that the laterite of the Seychelles Islands in the Indian
Ocean, derived from granitic, syenitic and trappean rocks, is not properly
a ferruginous clay, but a mechanical admixture of free quartz, iron oxide
and alumina hydrate in the form of hydrargillite.

CUMULOSE DEPOSITS

301

periods, but which have remained in a condition of slight con-
solidation, and through subsequent elevation and weathering
form the soils. Such vary widely and abruptly. In the region
northeast of Washington, the Potomac formation consists of
feldepathic sands, gravels, and clays irregularly bedded and
often enclosing notable accumulations of rounded pebbles of
quartzite brought from the Appalachian and Piedmont regions.
The Appomattox formation, from which was derived surface
soil in the vicinity of the Rappahannock and Appomattox in
Virginia, is a yellowish or orange-colored clay and sand with
sometimes interbedded gravel. The Columbia formation which
yields the surface soil of the main portion of Washington City
and the immediate valley of the Potomac and tributary streams
southward, is a delta and littoral deposit made up of materials
worked over from the older Potomac and Lafayette formations
and also of granitic sands and clays from the decomposed rocks
of the Piedmont plateau.

The clays of the Potomac formation above mentioned are at
times sufficiently homogeneous and plastic to be utilized in the
manufacture of brick, tiles, and pottery. The following table
shows the finely comminuted condition of the materials which go
to make up these clays in Maryland, as determined by Whitney. 1

MECHANICAL ANALYSES OF POTOMAC CLAYS, MARYLAND

DIAMETER

KM.

CONVENTIONAL NAMES

RKD CLAY,
TILE

RED CLAY,
PUDDLING

BLUE CLAY,

STONEWARB

2-1

Fine gravel

00 %

31 %

00 %

1-.5

Coarse sand . ....

0.00

082

000

.5-.25

Medium sand

0.50

2.69

029

.25-.!

2.63

3.23

1.27

.1-05

Very fine sand

962

889

893

.05-.01

Silt .

25.13

26 17

20 16

.01-.005

Fine silt . . .

13.44

11.18

16.72

.005-.0001

Clay

42.34

42.36

50.02

Total
Organic matter, water loss . .

93.66%
6.34

95.65 %
4.35

97.39 %
2.61

(2) Cumulose Deposits. To be classed with the sedentary
deposits, in that they result from the gradual accumulation of

1 Bull. 4, IT. S. Dept. of Agriculture, 1892.

302 THE EEGOLITH

material in situ, though differing radically in both composition
and origin from those just described, are those portions of the
regolith which result from the gradual accumulation of organic
matter with only small amounts of foreign detritus; which are
made up almost wholly of the combined accumulations, organic
and inorganic, of growing plants. Such may be found in all
stages of formation, in enclosed ponds or lakes, without appre-
ciable inlet or outlet, being merely due to standing water in
low places. ' * Such pools, when not exposed to periodical drying
up, are invaded by a peculiar vegetation, first mostly composed of
conferva?, simple thread-like plants of various color and of prodi-
gious activity of growth, mixed with a mass of infusoria, animal-
cules, and microscopic plants, which, partly decomposed, partly
containing the floating vegetation, soon fill the basins and cover the
bottom with a qoating of clay-like mould. So rapid is the work
of these minute beings, that in some cases from 6 to 10 inches
of this mud is deposited in one year. Some artificial basins in
the large ornamental parks of Europe have to be cleaned of such
muddy deposits of floating plants, mixed with small shells, every
three or four years.

"When left undisturbed, this mud becomes gradually thick
and solid; in some cases, of great thickness; affording a kind of
soil for marsh plants, which root at the bottom of the basins or
swamps and send off their stems and leaves to the surface of
the water or above it, where their substance becomes in the
sunshine hard and woody.

"As these plants periodically decay, their remains of course
drop to the bottom of the water; and each year the process is
repeated, with a more or less marked variation in the species
of the plants. After a time the basins become filled by these

FIG. 30. Section across a small lake, a, bed rock; 66, drift; cc, growing
peat; dd, decaying peat; ee, climbing bog.

successive accumulations of years or even centuries, and the
top surface of the decayed matter, being exposed to atmospheric
action, is transformed into -humus and is gradually covered by

CUMULOSE DEPOSITS 303

other kinds of plants, making meadows and forests. In other
cases when basins of stagnant water are too deep for vegetation
of aquatic plants, nature attains the same result by a different
special process; namely, by the prolonged vegetation of certain
kinds of floating mosses, especially the species known as sphagna.
These grow with prodigious speed, and expanding their branches
in every direction over the surface of ponds or small lakes, soon
cover it entirely. They thus form a thin floating carpet, which
as it gradually increases in thickness serves as a solid soil for
another kind of vegetation, that of the rushes, the sedges, and
some kinds of grasses, which grow abundantly mixed with the
mosses, and which by their water-absorbing structure furnish
a persistent humidity sufficient for the preservation of their
remains against aerial decay. The floating carpet of moss be-
comes still more solid, and is then overspread by many species
of larger swamp plants, and small arborescent shrubs, especially
those of the heath family; and so, in the lapse of years, by the
continual vegetation of the mosses, which is never interrupted,
and by the yearly deposits of plant remains, the carpet at last
becomes strong enough to support trees, and is changed into a
floating forest, until, becoming too heavy, it either breaks and
sinks suddenly to the bottom of the basin, or is slowly and grad-
ually lowered into it and covered with water." 1

It is to such processes that are due, in large part, the inland
swamp soils of many localities. Beginning at and near the
shore and upon a soil of wet sand, the organic matter has accu-
mulated year by year till now several feet in thickness and in
some cases covering miles of territory. The proportion of or-
ganic matter in such a deposit naturally increases from the shore
outward until in the upper and central layers it may comprise
90% of the total weight. I

This feature is well brought out in the following analyses
of material from an open ground prairie swamp in Carteret
County, North Carolina.

1 Geol. Survey of Pennsylvania, 1885, p. 106. The water hyacinth so
prolific in the sluggish streams of Florida, would, in time, doubtless pro-
duce similar conditions.

304 THE KEGOLITH

CHEMICAL ANALYSES OF SWAMP DEPOSITS, NORTH CAROLINA

CONSTITUENTS

Silica (insoluble) (SiOa)

80.84%

1.52%

Silica (soluble) (Si0 2 )

3.70

0.00

2.69

0.39

Oxide of iron (Fe20a)

1.18

0.15

Lime (CaO) ...

0.44

0.36

0.22

0.14

Potash (K 2 0)

0.07

0.06

Soda (Na 2 0)

0.02

0.13

Phosphoric acid (PgOs) . ....

0.08

0.06

Sulphuric acid (SOs)

0.06

0.00

Chlorine (Cl)

Trace

0.02

Organic matter (C)

7.70

87.25

Water (H20) ....

2.50

9.60

99.50%

99.68%

Column I of the above is from the margin the oak fringe
of this great swamp, near North River, about 8 miles north of
Beaufort; it is light gray to ash-colored with a growth of white
oak, gum, maple, pine, and palmetto trees; the situation is low
and flat. "This margin belt of semi-swamp is from a half mile
or less in width to above a mile. The surface rises towards the
interior and is covered by a soil, if it may be called such, repre-
sented by column II, which is 2 to 3 feet deep and upwards, and
lies on a bed of white sea-sand. It consists of a loose open mass
of half-decayed woody matter, of a brown color, and is in fact
a superficial, uncompressed lignite; for it will be observed that
the analysis includes nearly 10% of water, so that the dry sub-
stance would give but 3-|% of inorganic matter, not more than
would be accounted for by the ash of the woody matter. The
growth is a dense thicket of spindling shrubs with small scat-
tered maples and bays. ' ' *

Wiley has described 2 deposits of a somewhat similar nature
as covering 1,000,000 acres in the Kissimmee valley of Florida.
These, which are of a dark brown to deep black color, contain
in some cases as much as 96.16% of organic and volatile matter,
and vary from 3 to 20 feet in depth. Such, when properly

1 Geology of North Carolina, Vol. I, 1875.

2 Agricultural Science, Vol. VII, No. 3, 1893, pp. 106-120.

CUMULOSE DEPOSITS 305

drained, may be made extremely fertile, though in periods of
drought endangered by fire which, once started, may burn for
months, doing immense damage. The partially reclaimed areas
of the Great Dismal Swamp of Virginia are fairly representative
types of swamp soils.

The formation of cumulose deposits is not, however, limited
to lakes, stagnant ponds, or even to swamps as the word is ordi-
narily used, excepting as the swamp itself may be incidental
and consequent. Regions of poor drainage, particularly in
moist and cool climates, may give rise to growths of sphagnous
mosses and subsequently to plants of a higher type, which in
course of years assume no insignificant proportions.

In accounting for such accumulations, we have but to remem-
ber that ordinarily when a plant dies, its organic constituents
are returned to the atmosphere once more in a comparatively
brief period of time through the usual processes of decay. It
needs only such conditions of moisture as shall prevent com-
plete decay and hence favor the accumulation of the organic mat-
ter, to give rise to beds of peat and ultimately of coal. Plants
of the type of sphagnous mosses, growing continuously above
and dying beneath, hold in their mass sufficient moisture to
exclude atmospheric air, and thus themselves bring about the
proper conditions for bog making. In virtue of this property
such bogs may gradually rise above the level of the surrounding
country, as is the case with the Great Dismal Swamp of Vir-
ginia and numerous others that need not be mentioned here.
Instances are on record where bogs of this nature have grown
so far above the natural level, that during seasons of unusual
rainfall they have burst, and flooded adjacent regions, with dis-
astrous results. The rate of growth of such accumulations is
naturally quite variable. H. S. Gesner, as quoted by T. Rupert
Jones, 1 states that in Bavarian moors the observed increase in
peat, in forty-five years, amounted to from 2 to 3 feet in thick-
ness; in Oldenberg, in one hundred years, to 4 feet; in Ham-
melsmoor, Denmark, to 2^ feet ; and in Alpine districts to 4 and
5 feet in from thirty to fifty years.

The peat bogs, so characteristic of Ireland, Scotland, and
other northern latitudes, are of this type. A section of the

^roc. Geologists' Association, Vol. VI, No. 5, January, 1880.
21

306 THE KEGOLITH

well-known Bog of Allen, made in county Kildare, is given
below. 1

THICKNESS

(1) Dark reddish brown; mass compact; no fibres of moss visible;

surface decomposed by atmosphere . 2 feet

(2) Light reddish brown; fibres of moss very perfect 3 "

(3) Pale yellowish brown; fibres of moss very perceptible . . . 5 lt

(4) Deep reddish brown; fibres of moss perceptible .... 8^ feet

(5) Blackish brown; fibres of moss scarcely perceptible, contains

numerous twigs and small branches of birch, elder, and fir . 3 ' '

(6) Dull yellow-brown; fibres not visible; contains much empyreu-

matic oil; mass compact 3 "

(7) Blackish brown; mass compact; fibres not visible; contains

much empyreumatic oil 10 lt

(8) Black mass, very compact; has a strong resemblance to pitch

or coal ; fracture conchoidal in all directions ; lustre shining .4 ' '

Total depth of bog 38J feet

Underlaid by 3 feet of marl containing 64% carbonate of lime, 4 feet of
blue clay, and this in its turn by clay mixed with limestone gravel of an
unknown thickness.

1 T. Kupert Jones, Proc. of the Geologists ' Association, London, Vol. VI,
No. 5, January, 1880. This authority classifies the peat bogs, swamps, and
marshes, as follows:

I. Peat bogs and turf moors on such plateaux as flat mountain tops and
wide hill moors.

II. Peat bogs of valleys: (1) At the heads of valleys; (2) at the salient
angles within river curves; (3) in deserted beds of rivers; (4) in plains
and lakes of expanded valleys; (5) special peat bogs of Denmark and the
black earth of Kussia; (6) river deltas; (7) maritime peat marshes, where
certain valleys and plains open to the sea.

Eegarding the black earth of Eussia, it should be stated that this is now
regarded by at least one authority (Hume, Geol. Mag., Vol. I, No. 2, 1894)
as being but a local phase of the loess, the color being due to the preva-
lence of organic matter.

Shaler (Ann. Eep. U. S. Geol. Survey, 1888-89), on a basis of physical
characters, classifies the inundated lands of the United States as below:

M _ r . .. f Above mean tide . . f Grass marshes.

Marine marshes ( Mangrove marshes.

'-Below mean tide . . f Mud banks.

\ Eel-grass areas.
'River swamps . . . f Terrace.

\ Estuarine.

Lake swamps . . / Lake margins.

Fresh-water swamps . . .-J t Quaking bogs.

Upland swamps . . / Wet woods.

\ Climbing bogs.
Ablation swamps.

COLLUVIAL DEPOSITS 307

Deposits of the cumulose type pass by all gradations into
the paludal, swamp, or marsh type and these in turn into ordi-
nary alluvium. Or it would perhaps be better to reverse this
order, since, as in the gradual silting up of an enclosed lake,
we may have, in the first stages, stratified alluvium, then when
the waters become sufficiently shallowed, swamp and muck
deposits, and lastly the deposits of pure organic, or cumulose
material.

2. TRANSPORTED MATERIALS

Because of the constant action of gravity, the well-known
transporting power of water, the wind or moving ice, few re-
sidual products retain for any length of time their virgin purity,
but become more or less contaminated with materials from near
or distant sources. The avalanches of mountain regions afford
an illustration of the bodily transfer of, it may be, millions of
tons of matter from the mountain slopes to be debouched into
the valley below; the slow-creeping glacier brings down its
load and deposits its moraine when, succumbing to the blan-
dishments of warmer climes, it is no longer able to bear it fur-
ther : spasmodic winds catch up the smaller particles as clouds of
dust to be transported, assorted, and redeposited, as their force is
spent. It is, however, through running streams, both in the past
and present, and moving ice in ages gone, that has been brought
about the great amount of transportation and admixture charac-
teristic of that part of the regolith comprised under the general
name of drift. According to which of the agencies enumerated
prevailed, the resultant products may be classified as follows:
(1) Colluvial deposits, (2) alluvial deposits, (3) aeolian de-
posits, and (4) glacial deposits, though it will be found that the
lines of separation are not in all cases sharply drawn, and in
many an area the regolith bears impress of compounded agencies.

(1) Colluvial Deposits. 1 Under this head it is proposed to
include those heterogeneous aggregates of rock detritus com-
monly designated as talus and cliff debris. The material of
avalanches may also be classed here. Such result from the trans-
porting action of gravity. The deposits in themselves are com-
paratively limited in extent, ever varying in composition, and are

1 From the Latin ' ' colluvies, ' ' a mixture. The term as here used is more
restricted in its meaning than as defined by Professor Hilgard.

308

THE EEGOLITH

composed of an indiscriminate admixture of particles of all
sizes, from those as fine as dust to blocks it may be of hundreds
of tons' weight Such are necessarily limited to the immediate
vicinity of the cliffs or mountains from which they are derived.
As loosened by heat or frost from the parent masses, the frag-
ments tumble down the slopes, gradually accumulating in beds
the inclination of which is limited only by the laws of
gravity and the character of the debris (See PI. 25.)
Inclinations of 30 are common. From their
mode of origin it is natural that the individ-
\ ual particles should be mainly angular and
comparatively fresh. In fact, they represent
rock-weathering through disintegration, and
not decomposition, which will come later.
Above, they consist simply of masses of
loose rock wholly unfitted for the support
of vegetable life ; below, they pass gradu-

Soil "bearing portion
^

^oilless portion,

w///////7jm^

FIG. 31. Diagram
showing the history
of a talus, a, bed
rock; fcfc, talus; c, de-
stroyed portion of a cliff,
the material being now in
the talus.

ally into soils. (Fig. 31.)
Through becoming saturated with

water, ice, or snow, such at times become loosened from
the steep slopes on which they lie and slide down in the form of
avalanches into the valleys. (PL 27.) Although comparatively
limited in their extent, these latter, owing to the resistless
energy and suddenness of their advance, are sometimes appall-
ingly destructive, as has been repeatedly illustrated in mountain
regions the world over. The geographic distribution of talus
deposits as controlled by climatic conditions has been already
noted.

(2) Alluvial Deposits. The deposits included under this
head differ structurally from those thus far described in that
they are always more or less distinctly stratified, or bedded.
In writing of the formation of sedimentary rocks, and again

ALLUVIAL DEPOSITS 309

when treating of the action of running water, a few figures
were given relative to the amount of transported debris de-
posited yearly in the Gulf of Mexico In a similar way the
amount of debris carried annually to the ocean by some of the
chief rivers of the world has been estimated as below:

CUBIC FEET

Mississippi .... 7,468,694,400
Upper Ganges . . . 6,368,077,440
Hoang-Ho .... 17,520,000,000

CUBIC FEET

Khone 600,000,800

Danube 1,253,738,600

Po 1,510,147,000

The muddy condition of the water of certain rivers, caused
by this suspended matter, is so conspicuous a feature as to have
found recognition in the name applied. Hwang-Ho means simply*
yellow river; Missouri is the Indian name for Big Muddy; while
the famous Ked River of the North is so called merely because
of the red mud it carries. Such silt-bearing streams, flowing'
into lakes and tideless seas, begin depositing their loads so soon
as their currents are checked, building up thus the so-called
delta deposits for which the Mississippi, the Po, Ganges, and the
Nile are noted.

The character of the material in the delta deposits is vari-
able only within certain limits, consisting always of siliceous
sand and mud intermingled with organic matter.

Professor J. W. Judd found the materials of the Nile delta to
vary abruptly in texture from the surface downward, the varia-
tions following no recognizable law. The percentage amounts of
constituents classed as sand and mud, as obtained from (I)
borings at Kasr-el-Nil, Cairo, (II) Kafr-ez-Zayat, and (III)
Tantah, are given in the table on the next page.

The material described as sand consists of rounded, angular,
and sub-angular grains. The well-rounded granules are mainly
of quartz and feldspar; the angular and sub-angular of quartz,
feldspars, hornblende, and augite, with smaller quantities of
mica, tourmaline, sphene, iolite, zircon, fluor-spar, and magnetite
all in a nearly unaltered condition. The feldspars are mainly
orthoclase and microcline rarely a soda-lime variety and
in a state of surprising freshness. The quartz is in part the
quartz of granitic rocks and the larger grains well rounded,
best described as microscopic pebbles. "It is evident that
these sand grains have been formed by the breaking up of granitic
and metamorphic rocks, or of older sandstones derived directly
from such rocks. The larger grains exhibit the perfect rounding

310

THE BEGOLITH

MECHANICAL ANALYSES OF NILE DELTA DEPOSITS

i ii in

DEPTH

SAND

MUD

SAND

MUD

SAND

MUD

3'0" . . .

2.35

Of
10

97.65

% .

4'0" . . .

....

30.42

69.58

1.71

98.29

6' 0" ...

5.77

94.33

....

8' 6"

7.27

92.73

11' 0" . . .

....

50.99

49.01

16' 0" . . .

86.27

13.73

....

17' 6" . . .

79.65

20.35

....

18' 0" . . .

....

...

8.78

91.22

19' 0" . . .

....

87.41

12.59

....

22' 6" . . .

....

31.16

68.44

26' 0" . . .

90.19

9.81

31' 0" ...

....

39.43

60.57

35' 0" . . .

....

86.42

13.58

....

38' 6" . . .

65.05

34.95

....

...

....

40' 0" . . .

....

81.94

18.06

80.70

19.30

40' 6" ...

80.83

19.17

....

45' 0" . . .

68.72

31.28

....

46' 0" . . .

....

95.90

4.10

48' 0" . . .

....

87.23

12.77

65' 0" . . .

....

0.25

99.75

97.71

....

56' 0" . . .

....

99.53

2.29

68' 0" . . .

59.09

0.47

60' 0" ...

....

12.60

87.40

40.91

66' 0" ...

....

62.07

37.93

....

68' 0" ...

....

7.76

73' 0" . . .

....

59.95

92.24

75' 0" . . .

....

66.38

36.62

....

40.05

and polishing now recognized as characteristic of aeolian action ;
the smaller ones from their larger surfaces in proportion to their
weight, have undergone far less attrition in their passage through
the air ; but it is fair to conclude that they are really desert sand,
derived from the vast tracts which lie on either side of the Nile
valley, and swept into it by the action of the wind." The ma-
terial described as mud is composed of essentially the same ma-
terials as the sands, but in a more finely divided state. There
is an entire absence of anything like kaolin, though there are
present particles of organic matter and frustules of diatoms. The
surprising freshness of the materials and lack of kaolin Professor
Judd regarded as indicative of an origin through the action of

ALLUVIAL DEPOSITS 311

heat and frost; i. e., through mechanical agencies rather than
through the processes of rock decomposition. 1

But, as has already been noted, only a part of the sediment
carried by any stream reaches its mouth. A comparatively
small, but, from the present standpoint, very important portion
is carried during seasons of high water beyond the usual chan-
nels and spread out over the flood plains, as described on p. 276.

FIG. 32. Section across an alluvial plain.

Such deposits are plainly stratified, and consist of mineral mat-
ter in a finely comminuted condition derived, it may be, from
the breaking down of a great variety of rocks. Their physical
and chemical properties, as well as the periodic character of
their deposition, are favorable to the formation of soils possess-
ing great strength and fertility. Both fertility and rate of depo-
sition in such cases are augmented through plant growth, which
takes place with great rapidity wherever climatic conditions
are favorable. So soon as the water leaves the flood plain,
a host of moisture-loving plants, as reeds and rushes, spring up
in countless numbers to die down again in the fall, and yield their
carbon and nitrogenous constituents to serve as fertilizers,
and augment the crop of the following year. Moreover, the
remaining stems and fallen leaves serve to retard the running
waters of each succeeding flood, catching in their meshes the
floating sediments which might otherwise be carried seaward.
The Anacostia, which empties into the Potomac River east of
Washington, serves as a good illustration of the working of these
agencies. A century ago the stream was navigable by coasting
crafts as far as Bladensburg. Now, owing to shallow waters,
only rowboats can navigate beyond the Navy Yard at Washing-
ton. Each season the stream, murky with suspended silt from
cultivated fields along its shores, comes down, till, ponded back
by tides, it begins to deposit its load. As year by year its bed
is thus raised, water plants, encroaching more and more from

1 Proc. Eoyal Soc. of London, Vol. XXXIX, 1885, p. 213.

312 THE EEGOLITH

shallow shores, still further dam its sluggish current till now,
during summer months, it is little more than a stagnant pond
full of rank vegetation, and a source of odors foul and atmos-
pheres enervating. The so-called "Potomac Flats" south of the
city of Washington owe their origin and unhealthy conditions
to similar processes.

The method of alluvial deposition in the flood plain or delta,
of the lower Mississippi has been worked out by McGee, 1 whom
we cannot do better than quote in considerable detail.

In length this flood plain reaches from the mouth of the Ohio
1100 miles measured along the river, or half as far measured
in an air line, to the Gulf, and is bounded on the east by the
bluff rampart separating it from the contiguous district; it is
bounded on the west by a less continuous and less conspicuous
rampart crossing the Arkansas River at Little Rock and gradu-
ally failing southward until this district and its more westerly
neighbor nearly blend. The surface of this otherwise monoto-
nous district is relieved by a few small tracts of higher land.
Most conspicuous of these is Crowley Ridge in eastern Arkansas,
a long belt of upland stretching from the southeastern Missouri
southward between the White and St. Francis rivers to the
Mississippi at Helena. This belt of upland rises 100 or 200 feet
above the insulating flood plain, and in its steepness of slope
and rugosity of outline fairly simulates the eastern rampart
overlooking the "delta" in corresponding latitudes.

The vast lowland tract comprised in and constituting most of
this district is at once the most extensive and most complete
example of a land surface lying at base-level, or a trifle below,
that the continent affords.

It is trenched longitudinally by the Mississippi, and trans-
versely by the White, Arkansas, Red, and other large rivers;
between these greater waterways it is cut into a labyrinth of
peninsulas and islands by a network of lesser tributaries anjl
distributaries, the former gathering the waters from its own
surface and from adjacent country, and the latter aiding the
main river to discharge its vast volume of water and its immense
load of detritus into the Gulf. The whole surface lies so low
that it is flooded by periodic overflows of the Mississippi and
its larger tributaries, and with each flood receives a fresh coat-
ing of river sediment ; and much of the flood plain, fertilized by

1 The Lafayette Formation, Ann. Eep. U. S. Geol. Survey, 1890-91.

ALLUVIAL DEPOSITS 313

freshet deposits, is clothed with luxuriant forests and dense
tangles of undergrowth, or with brakes of cane, or with sub-
tropical shrubbery, only a few of the broader inter-stream tracts
being grassed. Partly by reason of this mantle of vegetation,
the current of each overflow is checked as the river rises above
its banks, and most of the sediment is dropped near by; and so
the Mississippi, the White, the Arkansas, and the Red, as well
as each lesser tributary and each distributary from the great
Atchafalaya down, are flanked by natural levees of height and
breadth proportionate to the depth and breadth of the stream.
The network of waterways is thus a network of double ridges
with channels between; and each inter-stream area is virtually
a shallow, dish-like pond in which the waters of the floods lie
long, to be drained finally, perhaps, through fresh-made breaks
in the natural dikes, weeks after the stream flood subsides. In
the southern part of the district the inter-stream basins approach
tide level and drain still more slowly ; in the sub-coastal zone
many of the basins are permanent tidal marshes. In the western
part of the district is an area in which the inter-stream basins
lie so high that they are invaded only by the highest floods and
veneered with only the finest sediments; in some cases these
sediments are so fine and so compactly aggregated and the
surface is so ill drained and watered that trees may hardly
take root, and these are either drowned by the floods or with-
ered by the sun in the drought. Such portions of the sur-
face are but scantily covered with coarse grass and form
the ''black prairies" of southern Arkansas and northwestern
Louisiana.

It is to similar processes as those described that the Nile valley
owes its remarkable fertility. The sediments deposited over the
plains during the season of freshets consist of fine sand brought
down by the Blue Nile and the Atbara from the decomposing
siliceous rocks of mountainous Abyssinia. The gneisses and gran-
ites yield their detritus to the lixiviating influence of the moun-
tain torrents and majestic Nile, the clayey particles being borne
seaward, while the fresh quartzose, feldspathic and other siliceous
particles, and smaller traces of apatite and alkaline carbonates
remain in just the right stage of subdivision to yield a soil,
which has brought forth for a period of over 4000 years crop
after crop without artificial fertilization.

The following table will serve to show the physical cnaracter-

314

THE KEGOLITH

istics of alluvial deposits, a portion of which are but reasserted
materials from the glacial drift.

APPROPRIATE NUMBER OF GRAINS OF SAND, SILT, AND CLAY IN ONE GRAMME
OF ALLUVIAL SUBSOIL FROM ILLINOIS

DIAMETER

CONVENTIONAL

(a)

(c)

MM.

NAMES

CHILLICOTHE

ROCKFORD

AMERICAN BOTTOMS

2-1

Fine gravel .

1-.5

Coarse sand .

.5-.25

Medium . , .

6,755

3,428

.25-.!

Fine sand . .

18,660

29,300

194

.1-.05

Very fine sand

53,470

212,400

151,400

.05-.01

Silt ....

4,670,000

5,888,000

12,230,000

.OJL-.005

Fine silt . .

86,860,000

115,100,000

195,600,000

.005-.0001

Clay ....

2,537,000,000

3,842,000,000

14,680,000,000

Total. . . .

2,628,608,968

3,693,233,177

14,887,981,599

(a) Terrace of Glacial age. (fc) Flood deposits,
race (bottom land of Mississippi).

The processes active in delta formation are manifested on a
smaller scale in the gradual silting up of many an inland lake,
particularly such as are of glacial origin.

It is a striking thought that all our lakes are but transient
enlargements of pre-existing streams, and will in time, per-
haps even before our own species is extinct, become converted
into broad expanses of meadow lands; and that our children's
children may yet sow and reap from rich and fertile areas which
now echo only to the cry of water-fowls, and the blue expanse
of which is broken but by wind-born waves and leaping fish.

The lithological character of the deposits thus formed vary
with certain limits almost indefinitely, since everything de-
pends on the character and quantity of the silt brought down
by the streams. Rarely, if ever, are they clayey, since the finer
particles are carried beyond. In nearly all instances they are
found to consist of very fine sand, largely siliceous, permeated,
often quite blackened, through the presence of organic matter.
Such are the mucks or mucky soils of New England.

So abundant is this organic matter, that when dried, such
deposits are used locally for mulching purposes, though in their
fresh condition they are sour and almost worthless except for
growing sedges and the ranker kinds of forage grass. During
the later stages of the process of filling up, deposition of sedi-

ALLUVIAL DEPOSITS 315

merits may almost entirely cease, since the water no longer rises
above the level of past accumulations. In such cases the final
stages consist simply in the accumulation of organic matter and
the deposits come to closely resemble, or are even superficially
identical with, the cumulose deposits already described. This
same statement holds good also for the closely related salt-water
marsh or paludal deposits, to be noted later,

Loess and Adobe. Under the head of transported deposits
must also be considered the so-called loess of the Mississippi val-
ley in our own country; of the Rhine valley, and other parts
of Europe; of northern China and the Russian steppes, though,
as will be seen, the name includes deposits which, while having
many physical properties in common, may vary widely in com-
position as well as in method of deposition. It is more than
doubtful, indeed, if the name has not been so loosely applied as
to rob it of its proper geological significance.

The loess of China, made famous through the researches of
Richtofen, is now regarded by some authorities 1 as of the same
nature as our adobe. Richtofen himself, it will be remembered,
regarded the Chinese loess as an asolian deposit, as due to the
action of wind in transporting for long distances the fine detritus
swept by rain and wind from mountain slopes into enclosed
basins, to ultimately become entangled and deposited among the
growing vegetation. This material, intermingled with the col-
lective residue of herbaceous plants, with the inorganic residuum
from the decay of prairie vegetation for countless generations,
makes up the mass of the loess over many hundreds of square
miles of territory, and in places to depths of thousands of feet.
The characteristics of the loess, as found in China, are those of a
fine calcareous silt or clay, of a yellowish or buff color, so slightly
coherent that it may be readily reduced to powder between the
thumb and fingers, and yet possessing such tenacity as to resist
the ordinary weathering action of the atmosphere, and, wherever
cut by stream erosion or other means, to stand with vertical walls,
even though they may be hundreds of feet in height. The loess
country is described as thus cut up by an almost impassable
system of gorges, so that to cross it in any fixed direction is
almost an impossibility. ''Wide chasms are surrounded by
castles, towers, peaks, and needles, all made up of yellow earth,

1 See I. C. Eussell, Subaerial Deposits of North America, Geol. Mag.,
August, 1889.

316 THE EEGOLITH

between which gorges and chasms radiate labyrinthically up-
wards into the walls of solid ground around. High upon a
rock of earth steeper than any rock of stone stands the
temple of the village, or a small fortress which affords the
villagers a safe retreat in times of danger. The only access
to such a place is by a spiral stairway dug out within the mass
of the bluff itself. In this yellow defile there are innumerable
nooks and recesses, often enlivened by thousands of people,
who dwell in caves dug in the loess. ' ' J

One of the striking features of the loess, both in China and
elsewhere, is the abundance of minute tubes or canals lined
with carbonate of lime which traverse it from above down-
ward, and which are assumed by some to be due to root fibres.
It is the presence of these presumably that causes the vertical
cleavage, and at the same time the remarkable absorptive quali-
ties for which the loess is noted. Such is the material which
for more than three thousand years has brought forth crops
continuously, and without exhaustion, over many square miles
of the Chinese Empire. Its distribution in Europe is given as
extending from the French coast at Sangatte, eastward across
the north of France and Belgium, filling up the depressions of
the Ardennes, passing far up the valleys of the Rhine and its
tributaries, the Neckar, Main, and Lahr; likewise those of the
Elke above Meissen, the Weser, Mulde, and Saale, the upper
Oder and Vistula. Spreading across upper Silesia, it sweeps
eastward over the plains of Poland and southern Russia, where
it forms the substratum of the tschernosem, or black earth.
It extends into Bohemia, Moravia, Hungaria, Galicia, Transyl-
vania, and Roumania far up into the Carpathians, where it
reaches heights of from 2000 to 5000 feet above sea-level. In
northern China it spreads over a large portion of the region
drained by the Hwang-Ho. For nearly a thousand miles
from the borders of the great alluvial plain of Pechele, through
the provinces of Shansi, Sensi, and Kansu, everywhere to the
northern base of the range of the Tsing-ling-shan, the loess
may be followed to the very divide which separates the basin
of the Hwang-Ho from the region destitute of drainage into
the sea. Toward the north it reaches almost to the edge of
the Mongolian plateau. The entire area covered continuously

ir The Chinese Loess Puzzle, by J. D. Whitney, American Naturalist,
December, 1877.

LOESS AND ADOBE

317

is stated to be as large as the whole of Germany, while it is
found in more or less detached portions over an area in addi-
tion, nearly half as large. In the United States the loess
covers thousands of square miles throughout the drainage
basin of the Mississippi River. It is found in Ohio, Indiana,
Michigan, Iowa, Kansas, Nebraska, Illinois, Tennessee, Ala-
bama, Mississippi, Louisiana, Arkansas, Missouri, Kentucky,
and the Indian Territory. According to Professor Aughey it
prevails over at least
three-fourths of Nebraska,
to a depth ranging from 5
to 150 feet, and furnishes
a soil of extraordinary
strength and fertility.

As here found, how-
ever, the Eeolian hypoth-
esis fails to satisfactorily
explain all the existing
conditions, and there is
little doubt but that it
represents in large part
the fine silt, the glacial
flour formed by the ice of

the Glacial epoch, borne FlG 33 ._ S howi^^tli^of particles in
southward by streams and Chinese loess,

deposited in water just

sufficiently in motion to carry the fine clay farther away. The
American loess, in fact, illustrates in a remarkable manner the
wonderful assorting power of water.

Microscopic and chemical examinations of loess sustain this
conclusion. The particles are as a rule quite fresh and sharply
angular. Out of 150,000 particles examined under the micro-
scope only about 3% measure above .0025 of a millimetre and
1% over .005 of a millimetre. Quartz is the preponderating
material, with lesser amounts of orthoclase and plagioclase feld-
spars, white and dark micas, hornblende, augite, magnetite,
dolomite, and calcite. The loess of the Rhine valley and of
China offers no differences that can be readily described, though,
as will be noticed by reference to the analyses, there may be a
wide difference in chemical composition. Indeed, the essential
characteristic of the loess is a physical rather than a chemical

318

THE KEGOLITH

r-lcO
COg(M

CO O CO r-t CO O
~ CN O 1-5 r4 ,-H

NA,

OI8

i iCOi i O CO to

LOESS AND ADOBE

319

one, and it is doubtless to this that is due its uniform fertility.
On p. 318 are given analyses of loess from the United States, the
Rhine valley, and from Switzerland.

The following table will serve to show the fine state of sub-
division in which the particles exist in loess as well as in a
dust brought down by snow, which will be described on p. 333.

MECHANICAL ANALYSIS OF LOESS AND DUST

III

CONSTITUENTS

UPLAND LOESS :
VIRGINIA CITY,
ILLINOIS

RIVER LOESS :
VIRGINIA CITY,
ILLINOIS

LOESS :
NEBBASEA

DtrsT FROM SNOW :
ROCKVILLE,
INDIANA

Moisture

5.40%

3 17 /

Organic matter .

4.96

11 10

11.98

0.00 %

0.00

Coarse sand
Medium sand

0.00
0.00

0.00
0.01

0.00
0.00

Fine sand ...

0.01

0.10

0.00

Very fine sand

7.68

24.84

23.14

0.00

Silt

61.85

60.98

54.81

69.37

Fine silt

9.60

2.80

2.46

5 80

Clay

15.15

6.15

9.45

9.68

94.29%

94.88%

99.22%

100.00%

Aughey 1 gives the following section of the loess and soil in

Nebraska.

(1) Loess . .4 feet

(2) Black soil 2 "

(3) Loess 4 "

(4) Black soil 1| "

(5) Loess 5 "

(6) Black soil 1* "

(7) Stratified loess . . . . . 15 "

This alternation is accounted for on the assumption of fre-
quent changes of level during the loess-forming period. It
would seem that the loess was deposited in shallow water and
that as the lake became filled plant life came in as in modern
bogs and marshes, and throve until sufficient organic matter was

1 Physical Geology and Geography of Nebraska, p. 276.

320 THE KEGOLITH

formed to make the black soil layer. A period of subsidence
followed, more loess was deposited and the previous condition
repeated, this process going on till all the layers were formed.

The name adobe is given to a calcareous clay of a gray-brown
or yellowish color, very fine-grained and porous, which is suffi-
ciently friable to crumble readily in the fingers, and yet, like
loess, has sufficient coherency to stand for many years in the
form of vertical escarpments, without appreciable talus slopes.
The material of the adobe is derived from the waste of the
surrounding mountain slopes, the disintegration being largely
mechanical. According to Professor I. C. Russell, 1 from whose
descriptions is drawn a portion of what is given here, it is as-
sorted and spread out over the valley bottom by the action of
ephemeral streams, where it becomes mixed with dust blown
by the winds from the neighboring mountains, and rendered more
or less coherent by the cementing action of interstitial carbonate
of lime.

Hilgard 2 limits the name adobe to the distinctly clayey soils of
the arid regions, and divides them into two classes, the upland
and the valley adobes, the first being derived mainly from the
disintegration, in place, of clay shales, while the second are
mostly paludal or swamp formations, and represent either the
finest materials that remain suspended in slack water, from any
source, or sometimes the direct washings of the clayey soils of
the hills. Whichever authority we follow, it is evident the
name includes materials alike not in mode of origin or com-
position, but only in physical characteristics.

Adobe forms the soil of a large portion of the rainless region
of the United States. It is found therefore in Colorado, Utah,
Nevada, southern California, Arizona, New Mexico, and west-
ern Texas, as well as in the southern portion of Idaho, Wyoming,
and Oregon. It has also a wide distribution in Mexico. In
the United States it occurs from near the sea-level in Arizona,
and even below it in southern California up to an elevation of
at least 6000 or 8000 feet along the eastern border of the Rocky
Mountains, and in the elevated valleys of New Mexico, Colorado,
and Wyoming.

The maximum thickness of the various deposits grouped
under this name is not in all cases readily determined, for the

1 Subaerial Deposits of North America, Geol. Mag., August, 1889.

2 Bull. 3, U. S. Weather Bureau, Dept. of Agriculture, 1892.

LOESS AND ADOBE

321

reason that it is still accumulating and has not been sufficiently
dissected by erosion to expose sections to any considerable
depth. Many of the valleys of the arid region have been filled
by it to a depth of 2000 or 3000 feet. In the larger valleys
there are rocky crests, called "lost mountains," which project
above the broad level desert surface, which are in reality the
summits of precipitous mountains that have been almost com-
pletely buried beneath these recent accumulations. The pre-
vailing color of adobe is light buff to gray, excepting when
contaminated with organic matter. In its typical form it is so
fine as to be quite without grit when rubbed between the fingers.
When examined under the microscope, it is seen to be com-
posed of irregular unassorted flakes and grains, principally
quartz, but fragments of other minerals are also present. The
adobe of Salt Lake shows flocculent masses* of amorphous matter,
which, when thoroughly disintegrated, are found to consist of
minute sharply angular fragments of quartz and feldspar with
much calcareous matter, and only rarely a shred of micaceous
or hornblendic material. In size the particles vary from those
too small for measurement up to .08 millimetre in diameter.

The valuable characteristics of the adobe are its extreme fine-
ness, great depth, and wonderful fertility.

CHEMICAL ANALYSES OF ADOBE

CONSTITUENTS

Silica (SiOg) .

66.69%

44 64 %

A lun linn f AlgOs) . .

14.16

13.19

Ferric oxide (Fe20s)

4.38

6.12

0.09

0.13

Lime (CaO)

2.49

13.91

Magnesia (MgO)

. 1.28

2.96

Potash (K 2 0) ... . .

1.21

1.71

Soda (NajO)

0.59

Carbonic acid (CO 2 )
Phosphoric acid (PgOs)

0.77
0.29

8.55
0.94

0.41

0.64

Chlorine (Cl)

0.34

0.14

Water (H 2 0)

4.94

3.84

Organic matter

2.00

3.43

99.72o/

99.79%

I. Adobe from Santa Fe, New Mexico. II. Adobe from Fort Wingate,
New Mexico.

322 THE EEGOLITH

Although comprising the soil of almost the entire region that
was but recently known as the Great American Desert, it needs
but water to make it laugh with harvests. While its physical
properties undoubtedly have much to do with its fertility, this
quality must also be in part due to the fresh and undecomposed
condition of its constituent parts. Originating doubtless by
purely mechanical agencies, it has been swept by winds and
spasmodic rains into closely adjacent basins occupied by but
temporary lakes, where, spread out over a floor sometimes almost
absolutely level, it has been subjected to a minimum amount of
leaching and retains until to-day its youthful strength and
powers of recuperation. 1 The analyses given on p. 321 will
serve to show the varying character of the deposits included
under this name. Especial attention need be called only to
the relatively high percentages of lime and the alkalies.

Champlain Clays. Under the head of alluvial deposits must
also be considered those clay accumulations which result from
the deposition of fine aluminous sediments sorted by running
streams from glacial debris and like the loess laid down in quiet
water, though usually estuarian rather than lacustrine. These
are the well-known Leda or Champlain clays 2 of glacial regions,
which on genetic grounds might well be classed as aqueo-glacial
deposits.

Such are very abundant along all the lower valleys of the
principal rivers of New England, sometimes coming to the im-
mediate surface or overlaid with a thin layer of sandy material
which, together with a little organic matter, forms the true soil.
They form, according to Dawson, 3 the sub-soils over a large part
of the great plains of Lower Canada, varying in thickness up
to 50 or even 100 feet, usually resting upon the boulder clay.
They are, as a rule, of almost impalpable fineness, unctuous, and
extremely plastic. Excepting where superficially oxidized to
buff or brown, they are of a blue-gray color and may show on
analysis considerable quantities of lime carbonate and alkalies,
features whereby they are readily distinguished from the resid-
ual clays, and which are regarded as indicative of an origin by
mechanical rather than chemical means. When dried, they be-
come greatly indurated, and when unmixed with other mate-
rials, bake so hard during seasons of drought, or are so plastic

1 See further on p. 357.

2 So called from their most characteristic fossil, Leda.

3 The Canadian Ice Age.

PLATE 28

FIG. 1. Section of beds of Leda clay, Lewiston, Maine.

FIG. 2. Beds of volcanic dust, Keese Creek, Gallatin County, Montana.

THE CHAMPLAJN CLAYS

323

during seasons of rainfall, as to be quite unsuited for cultivation.
Mixed with varying proportions of siliceous sand to counteract
shrinkage, they form the common brick-making materials of the
Northeastern states.

The materials of the Leda clays naturally vary in different
localities, being dependent on the characteristics of the rocks
from which they were de-
rived. Those of Canada,
according to Dawson,
were derived from the
waste of the Utica and
Quebec groups. This
authority believes that
when the clay was in sus-
pension, it was probably
of a reddish or brown
color from the iron per-
oxide it contained, tmt
that, like the bottom mud
now forming in the deeper
parts of the St. Lawrence,
the coloring matter be- FIG.
came deoxidized by or-
ganic matter so soon as
deposited, the sesquioxide
of iron being converted into sulphide or protoxide carbonate.
Inasmuch, however, as the materials were so largely derived
by the grinding action of the glaciers on fresh rocks, it is not
impossible that they may have been again deposited as clay
without having ever undergone the oxidizing process.

Unlike the till or boulder clays, these Leda clays are dis-
tinctly stratified, as shown in the accompanying illustration. (PI.
27.) An analysis of a sample from the locality figured yielded
the author results as given in column I on p. 324. In column II
is given that of the portion (33.56%) soluble in hydrochloric
acid and sodium carbonate solutions, while in column III is given
the composition of a ' ' semi-assorted glacio-lacustrine" clay
bordering on Lake Michigan near Milwaukee, Wisconsin, and
in IV a glacial pebbly clay underlying II at the same locality. 1

1 Analyses II and III from Chamberlin and Salisbury 's paper, 6th Ann.
Eep. U. S. Geol. Survey, 1884-85.

34. Showing particles from Leda
clays. 1, quartz; 2, orthoclase; 3, plagio-
clase ; 4, mica ; 4, tourmaline ; 6, pyroxene ;
7, chlorite; 8, hornblende.

324

THE KEGOLITH
CHEMICAL ANALYSES OF STRATIFIED CLAYS

CONSTITUENTS

ill

Silica (SiO 2 )

56.17 %

10.98%

40.22 %

48.81 %

24 25

8.66

8.47

7.54

Phosphoric acid (P 2 0g)

Not det.

0.05

0.13

Titanic oxide (Ti0 2 )

Not det.

0.35

0.45

Not det.

2.83

2.53

Ferrous iron (FeO)

3.54

5 19 1

Manganese oxide (MnO)

Not det.

Trace

003

Lime (CaO) . ...

2.09

1.02

15.65

11.83

Magnesia (MgO)

2.57

2.19

7.80

7.05

Potash (K 2 0)

4.06

1.12

2.36

2.60

Soda (Na 2 0)

2.25

0.75

0.84

0.92

Water (H 2 0)

4 69

3 65

1 95 2

2 02 2

Carbonic acid (C0 2 )

None

18.76

15.47

Organic carbon (C)

None

0.32

0.38

Sulphuric anhydride (S0 3 ) ....

None

0.13

0.05

99.62%

33.56%

100.21 %

100.46%

Salt-water Marsh, or Paludal Deposits. Related to the delta
deposits already described, but differing in that their inorganic
materials are in large part derived immediately from the sea, are
the salt-water marsh, or paludal deposits so common along the
Atlantic border of North America. In discussing the formation
of these and their gradual transitions into arable lands, one can-
not do better than follow Professor N. S. Shaler. 3

The formation of a sea-coast swamp is due mainly to wave
action and plant growth, and is dependent upon the configura-
tion of the coast. Wave action upon an irregular coast such
as that of New England nearly always results in a breaking or
wearing away of the exposed headlands and the transportation of
the debris into intervening inlets, where it is thrown upon,
or at least in a direction toward, the beaches. On these beaches
the rock fragments are ever being ground smaller and smaller,
and must in time be reduced to the condition of the finest sand
and mud. Each incoming wave hurls more or less of the frag-
mental material upon the beach, whence a considerable portion

1 A11 iron determined as FeO.

2 Contains H of organic matter dried at 100 C.

8 Ann. Rep. Director of the U. S. Geol. Survey, 1884-85.

SEA-COAST SWAMP DEPOSITS 325

of it may be again carried seaward by the bottom current or
undertow as the wave recedes. One who has stood upon a high
rock on the sea-shore and watched the waves come tumbling at his
feet and then go creeping oceanward once more cannot have
failed to notice the continual seething sound due to the drag of
the rock fragments as they are impelled inward and outward by
the alternating currents. A considerable part of the mud thus
formed is taken out to sea by the undertow 7 , which always sets
from a storm-beaten beach along the bottom, but another part is
urged by the movement of the water caused by the waves and the
tidal flow into the fjords, where it falls to the bottom. In this
process the mud is generally conveyed along the shores and most
commonly deposited in the parts of the inlets near the shore
line. Wherever there is a bay within which the tidal current
is deadened and where the waves have little play, the sediment
is most rapidly laid down. If the process of deposition begins
on a pebbly bottom, it is at first aided by the irregularities be-
tween the stones and the friction of the water among the sea-
weeds, which attach themselves to the stones. As soon as a
sheet of mud is established, it commonly becomes occupied by
a dense growth of eel-grass, which greatly favors the deposition of
sediment. The stems of the grass are set very closely together,
the interspaces generally not exceeding 1 or 2 inches. A
tidal current of 2 miles an hour, swift enough to carry much
sediment, is almost entirely deadened in this tangle of plants.

At half tide on the New England coast these eel-grass fields
are generally covered with water to the depth of several feet;
at this stage the tidal currents are commonly strongest. The
water above the level of the grass has its usual freedom of
motion and brings much sedimentary matter above the level of
the foliage. As the tide falls, a part of this waste is entangled
and held until it gradually sinks to the bottom, so that each
run of the tide gives a certain contribution of sedimentary
matter, which goes to shallow the water. The process is easily
observed from a boat floating over a field of these plants. The
deadening of the current when the lowered tide brings the tops
of the plants near the surface is very noticeable. The mass of
floating matter mud, fronds of sea-weed (often with shells or
small pebbles attached to their bases) , dead fish, and other refuse,
is seen to collect in the mesh of foliage and sink. The dead
stems of the eel-grass and the bodies of many small crustaceans

326 THE EEGOLITH

and mollusca which live on its stalks or on the bottom contribute
to the deposit, so that it thickens with considerable rapidity.

When the bed thus formed has risen to the point where it is
dry at low water of the ordinary run of tides, the eel-grass
can no longer maintain itself, but gives place to other groups
of sea-weeds and grasses. These plants find their place first
near the shore line, where the eel-grass platform is naturally the
highest. At first the vegetation is quite sparse, owing to the
difficulty with which they endure the depth of water at high tide.
There is often, indeed, a considerable difficulty in establishing the
growth of the second group of plants, and for a while the de-
posit takes the shape of bare mud-flats, dependent in the main
for their accumulation of detrital matter on the growth of
certain mollusca, especially of the genera Mytilus and Modiola.

PIG. 35. Cross-section of marine marsh, a, original surface of shore line;
5, grassy marsh; c, mud-flats; d, eel-grass; e, mud accumulated in eel-

When, as is usually the case, the more highly organized plants
have difficulty in establishing themselves over the broad surface
of the mud-flat, they win their way to it in the following manner.
From the vantage ground of the shore line, where are found
the conditions of submergence which suit their needs, the plants
slowly extend the front of their bench out over the mud-flats.
(See Fig. 35.) This process of growth can be more easily studied
than that of the earlier or eel-grass stage of the marshes, for it is
visible along miles of our sea-shore. The higher grasses have even
more thick-set stems than those of the eel-grass flats; they en-
tangle sediment even more effectively. At first their stems are
covered for a few hours at each ordinary tide; they gather
waste rapidly, and soon lift the plain which they are constructing
up to the point where only at the highest tides are the tops
covered by water. At this stage the growth of the deposit is
practically arrested, there being no means of increase save from
the decay of the grasses themselves.

SEA-COAST SWAMP DEPOSITS

327

"On the central parts of the New England shore, as about
Boston, the mud-flat occupies at most two or three feet in the alti-
tude above mean low tide and the annual addition to its mass
in a year is very small," perhaps not so much as the tenth of an
inch in a year. "On the other hand, in the Basin of Minas,
one of the principal inlets leading from the Bay of Fundy, the
contribution of sediment is so great that vast areas have been
easily reclaimed from the sea by building a rude enclosure
around an area of the higher parts of the mud-flat, so that the
speed of the sediment-laden waters is checked and they are
made to lay down their burdens. In a few years, often in a
few months, this enclosed area is raised to near the level of
high tides. It is then only necessary to erect a barrier suffi-
cient to exclude the tide, with gates for the rain water, in order
to have the land completely reclaimed from the sea. In this
simple way there has been an area of many thousand acres of
excellent arable land created along these shores." 1

CHEMICAL ANALYSES OF SEA COAST SWAMP DEPOSITS

CONSTITUENTS

Silica insoluble

64.42 %

72.70%

1.92

Oxide of iron and alumiini ....

16.45

5.69

Lime ....

1.18

1.39

Magnesia

0.07

0.05

Potash

1.18

1.82

Soda

0.79

0.35

Phosphoric acid

0.25

0.13

Sulphuric acid . . .

1.46

0.33

Organic matter

10.35

Water

20.92 1

Oxide of manganese

0.54J

3.65

Sulphide of iron

1.09

0.11

1.63

1.71

99.98 %

100.10%

1 As the total reclaimable area between New York and Portland (Maine)
probably exceeds 200,000 acres, their money value in their best state will
amount to at least $40,000,000. The cost of reclaiming these areas and
reducing them to cultivation should not exceed the fifth part of that sum.
It may be noted that from the chemical composition of these soils, they are
practically inexhaustible, and that from their position they are often well

328

THE EEGOLITH

The lithological and chemical character of deposits of this
nature have been but little studied, and we are here able to give
only the two analyses on p. 327, in which, however, it is probable
that the matter tabulated as insoluble silica includes as well all
silicates insoluble in acid.

Column I of the table is mud from the marshes of Newport
River, a few miles above Beaufort, in Carteret County, North
Carolina. This marsh, formed by the filling up of the old river
channel, several miles wide, is continually enlarging at the
expense of the water surface; similar formations, to the extent
of hundreds of square miles, are accumulating in very many
shallow bays and sounds and rivers near the sea. Column II is
the sea mud or slime which is deposited in the shoal waters of
Beaufort Harbor and along the sounds and estuaries of the North
Carolina coast. It is a fine, dark-colored salt mud, formed of the
silt brought down by the rivers, mixed with decaying vegetable
matter and animal remains. 1

MECHANICAL ANALYSES OF SWAMP DEPOSITS

DIAMETER

III

PARTICLES
mm.

CONVENTIONAL NAMES

SOIL
0-6 inches

SUB-SOIL
6-9 inches

SOIL
0-6 inches

SUB-SOIL
6-9 inches

2-1

Fine gravel

0.00%

0.00 %

1-.5

Coarse sand

0.71

0.08

1.36

0.14

.5-.25

Medium sand

2.70

0.25

5.18

0.43

.25-.!
.1-.05
.05-.01

Fine sand
Very fine sand ....
Silt . . .

0.83
0.37
10.32

0.13
0.15
13.97

1.59
0.71
19.79

0.23
0.26
24.30

.01-.005

Fine silt ... .

5.32

7.10

10.20

14.09

.005-.0001

Clay

31.90

34.85

61.17

60.65

Total mineral matter . .
Organic matter, water loss

52.15%
47.85

56.53%
43.47

100.00 %

Loss by direct ignition .

100.00 %
47.36

100.00 %
39.65

placed for irrigation. South of the New England shore the marsh area
is much more extensive than in that region. It is probable that the im-
provable marshes of the Atlantic coast amount to at least 3,000,000 acres
and they may exceed double this amount. (Shaler, p. 380.)
1 Geology of North Carolina, Vol. I, 1875, p. 214.

BEACH SANDS

What is described by Whitney 1 as a typical swamp, bog or
peat soil, from a rice field near Georgetown, South Carolina,
yielded the results given on the preceding page, columns III and
IV being simply recalculated from I and II on an organic and
water-free basis. These are the so-called sob-field soils, in them-
selves poor, but responding readily to fertilizers. When ex-
hausted by cultivation, they recuperate quickly through the aid
of silt deposits from the rivers, brought about by the continual
ebb and flow of the tides.

Beach Sands. Although differing radically in composition
from the sea-coast swamp deposits already described, one must,
on account of their intimate geological relationship, include here
a brief description of those fragmental deposits formed by wave
action along beaches and in many instances almost absolutely
free from organic matter of any kind. Such are the clean
white beach sands, the delight of the summer visitor at the sea-
sides. These are found here and there in isolated stretches
along the Atlantic slopes, particularly where, as at Old Orchard,
Maine, they receive the full sweep of wave and tide from the
open sea. In many instances the material forming these beaches
is siliceous sand from glacial deposits which the ocean has
reasserted according to its own liking. In other cases it is
sand brought down by rivers, which has undergone fractional
separation through the varying strength of transporting agencies.
In still others it is material derived immediately from the shore
rocks through the weathering action of atmospheres and the
hammering of the waves. In other cases yet, as along the coasts
of Florida, the source is problematical. It can only be said,
knowing the character of rocks forming the mainland, that they
could not have here originated, but must have been transported,
and probably down the coast, from the areas of crystalline rocks
to' the northward. It is sometimes, though not always, possible
to gain an idea of the probable source of these sands, through a
study of their mineralogical nature and the physical condition
of the individual particles.

Sorby, who devoted careful attention to the microscopic ap-
pearance of granules of quartz sand belonging to various geo-
logical periods, divided them into five types, "which though

'Rice, Its Cultivation, Production, and Distribution, Rep. No. 6, Misc.
Series, U. S. Dept. of Agriculture, 1893.

330 THE KEGOLITH

characteristically distinct, gradually pass into one another/' 1
These types are :

1. Normal, angular, fresh-formed sand, as derived almost
directly from granitic or schistose rocks.

2. Well-worn sand in rounded grains, the original angles
being completely lost, and the surface looking like fine ground
glass.

3. Sand mechanically broken into sharp angular chips, show-
ing a glassy fracture.

4. Sand having the grains chemically corroded, so as to pro-
duce a peculiar texture of the surface, differing from that of
worn grains or crystals.

5. Sand in which the grains have a perfect crystalline out-
line, in some cases undoubtedly due to the deposition of quartz
over rounded or angular nuclei of ordinary non-crystalline sand.

The material of most beach sands is largely quartz, though
not invariably so. Those of the Bermudas and other coral
islands are, as a matter of necessity, calcareous. Those of isolated
deep-sea islands like the Hawaiians, are derived in part from the
volcanic rocks of the islands, and in some instances are composed
almost wholly of minute shells of the size of a pin's head. These
last from their faculty of emitting a crunching sound when dis-
turbed, are known as "sounding" or "singing sands."

The beach sand at Diamond Head, Oahu, is mainly of olivine
and magnetite granules, with smaller amounts of calcareous
matter. As usual, the grains in samples from the same level
are of fairly uniform dimension, varying from 0.5-1.0 milli-
metre, the larger forms being often fairly well rounded, while
the smaller may still show crystal outlines. The granules, even
in the same sample, however, vary greatly in the amount of
rounding they have undergone. Like the quartz granules from
the Florida beach next to be noted, these show conchoidal chip-
pings due to the shock of impact as one granule strikes against
another.

The beach of Santa Rosa island, south of Pensacola, Florida,
is composed of clear white quartz sand of almost ideal purity.
The grains, though water-worn and with the lesser angles
rounded, are still in many cases angular, and of very uniform
size (about .5-1.0 millimetre), as shown in Fig. 36. These
granules offer a very beautiful illustration of Sorby's type No.

1 Proc. Geol. Soc. of London, Anniversary Address, Session, 1879-80, p. 58.

BEACH SANDS

331

2, the surface of each one, through abrasion, being reduced to
the condition of ground glass. Examination with a high power
brings out minute fractures and conchoidal chippings, at once
suggestive of the preliminary stages of manufacture of the
quartz spheres for which the Japanese are so noted. It is as
though each granule had been held in the hand of some pigmy
aboriginal, and its surface reduced by hammering with another
pebble, after the manner known among archaeologists as
"pecking."

The shape assumed by a rock or mineral fragment subjected
to wave action varies somewhat with the nature of the material,
schistose rocks and easily cleavable minerals naturally giving
rise to pebbles or granules of quite unequal dimensions in three
directions. The schist on the coast of Cape Elizabeth, Maine,
for instance, gives rise to
pebbles in the form of a
greatly flattened oval,
while the more homo-
geneous quartz, with
which it is associated,
yields nearly spherical
forms. But of whatever
character the material,
the normal shape of a
beach-formed boulder or
pebble is oval, and this
for the reason that the
wave action is a dragging
rather than a carrying
one ; the stone is not lifted
bodily and hurled toward
the shore to roll back with
the receding wave, but is rather shoved and dragged along.
Gravity tends to hold the fragments in one position so that
the wear is greatest on the side which is down, and this in
itself would cause them to assume an oval or flattened form
even were they spherical and of homogeneous material at the
start." 1

(3) /Eolian Deposits. We will now consider those deposits

1 Merrill, Preliminary Handbook, Dept. of Geology, U. S. National Mu-
seum, 1889, p. 23.

FIG. 36. Quartz granules in sand from
beach, Santa Rosa Island.

332 THE EEGOLITH

which owe their origin and present structural features mainly
to wind action, though, as is made apparent in the discussion of
the loess and adobe, sharp lines cannot in all cases be drawn
between such and those of alluvial origin.

The efficacy of the wind as an agent of transportation was
dwelt upon in considerable detail on pp. 163 and 280. The
material thus carried into the air, often to great heights, is
brought to the surface again by gravity, though the normal rate
of descent is frequently greatly accelerated by rain or snow.
Indeed, the clearness, limpidity, of the atmosphere after a rain-
fall is due simply to the fact that it has been washed, is cleansed
of its suspended impurities.

The amount of dust brought down even from moderately clear
atmospheres is often sufficiently abundant to attract the at-
tention of the most casual observer. Professor H. L. Buner of
Irvington, Indiana, has stated 1 that during a storm in Febru-
ary, 1895, a layer of snow about one-fourth of an inch in thick-
ness was colored distinctly brown by the dust it contained.
One sample of snow collected yielded .37% of dust, by weight,
and it was calculated that the material was thus deposited at the
rate of 30.7 pounds avoirdupois for each acre. Another observer
calculated the fall as taking place at the rate of 12.77 pounds
per acre.

From a gallon of water melted from a snowfall of but 4
inches, which fell in London in January, 1895, there was obtained
10.65 grains of solid matter, 5.75 grains being inorganic and
4.90 grains carbonaceous. Water from a snow collected near
the centre of the city, January 30 of this same year, gave 6.25
grains of mineral and 11.07 grains of carbonaceous matter. It
was also found that 75% of these impurities were brought down
with the first 2 inches of the snowfall.

Dr. Whitney examined samples of the black earth brought
down near Rockville, Indiana, during a snowfall of the win-
ter of 1895 and reported it as consisting of material almost
identical with the prevailing loess of that region, from whence
it was doubtless derived. The individual particles varied in size
between .10 and .05 millimetre. The results of a mechanical
analysis of the dust are tabulated with those of loess on p. 319.
Samples of the same dust submitted to microscopic examination

1 Monthly Weather Review, U. S. Dept. of Agriculture, January, 1895.

AEOLIAN DEPOSITS

333

were found to consist of fully 96% silt and 4% organic matter,
the latter consisting mainly of fresh-water algae, diatoms, fungi,
cells from decayed grasses, and shreds of woody tissue.

CHEMICAL ANALYSES OF DUST SOILS

CONSTITUENTS

ATATHNAM
PRAIRIE,
YAKIMA
COUNTY,
WASHINGTON

RATTLESNAKE
CREEK, KITTI-
TAS COUNTY,
WASHINGTON

PLATEAU ON
WILLOW CREEK,
MORROW
COUNTY,
OREGON

Insoluble matter

%
71 67")

%
7833)

%
7921 )

Soluble silica ...

5 11 1 76 ' 78

2 20 1 80 " 63

2 30 } 81.51

Potash (K 2 0)

1.07

0.70

0.89

Soda (Na 2 0)

0.35

0.24

0.05

Lime (CaO)

200

208

137

Magnesia (MgO)

134

147

1 08

Brown oxide of manganese (Mn 3 O 4 ) .
Peroxide of iron (Fe2Og)
Alumina (AljOs)

0.04
6.88
7.91

0.07
6.13
6.12

0.06
5.63
6.02

Phosphoric acid (PaOg)

Sulphuric acid (SOa)

002

003

Water and organic matter ....

2.82

2.35

2.55

Total

99 34 %

99 89 %

99.37%

Humus

4.10

0.44

Hygroscopic moisture

498

320

492

MECHANICAL ANALYSES OF DUST SOILS

CONVENTIONAL NAME

DIAMETER OF
PARTICLES

III

Clay .

.0023 mm.

0.93%

3.59%

1.27 %

.005-.011 '

30.93

13.06

32.29

Silt

.013-.027

3.20

5.82

12.75

Very fine sand

.027-.05 '

7.18

27.37

37.51

.05-. 122

21.88

43.78

10.92

.122-. 5

32.39

4.57

3.97

96.51 %

98.19%

98.71%

Hilgard has examined the so-called "dust soils'* of Oregon,
California, and Washington, which during the dry seasons are

334 THE KEGOL1TH

so loose and fine as to rise in clouds at the merest puff of
wind, and gives the tables on the preceding page to show their
chemical and physical natures. These he regards as fairly typ-
ical for soils of the arid regions of the United States. 1

Sand Dunes. The influence of the wind in the formation
of sand hills or dunes, as they are commonly called, has received
attention on p. 163. A few words more regarding their physical
qualities and lithological nature are here essential.

The effect of the single whirlwind or it may be that of the
more constant air current for days, weeks, or even months,
may be from a geological standpoint comparatively insignifi-
cant; but they are, nevertheless, interesting, and at times
important. In certain regions of the West, and notably in
parts of the Colorado desert, as described by W. P. Blake, in
1853, all the fine loose sand on the surface of the ground is
blown away, leaving every pebble and boulder standing out in
strong relief from the hard sun-baked soil, or ledge of bed-rock.

Under favorable conditions the material thus blown along
may gather in the form of dunes, which themselves travel
slowly across the country, ever changing their outlines like
drifts of snow. A few miles north of Winnemucca Lake, in
western Nevada, is a belt of these dunes described by geologist
Russell 2 as fully 75 feet in thickness and about 40 miles in
length by 8 miles in breadth. These, under the restless
goading of the winds, are constantly varying in shape, and
though moving in mass probably but a few feet a year have
already, in more than one instance, made necessary the splicing
of telegraph poles to prevent the burial of the wires. Another
range of sand dunes, at least 20 miles in length, and forming
hills 200 to 300 feet high, occurs on the eastern end of Alkali
Lake in the same state. On the eastern shore of Lake Michi-
gan are also dunes of sand sometimes 200 feet in height, and
which at Grand Haven and Sleeping Bear have drifted over
the adjacent woodlands, leaving only the dead tops of trees
exposed. Similar dunes occur frequently on the Atlantic
coast, as at Hatteras, Long Island, and Cape Cod. The island
of Bermuda is made up almost altogether of coral and shell
fragments. These are washed by the waves upon the beaches,

iBull. No. 3, Weather Bureau, U. S. Dept. of Agriculture, 1892.

* Geological History of Lake Lahontan, Monograph XI, U. S. Geol. Sur-
vey, 1885.

DEPOSITS 335

dried by the winds, and blown gradually inland, thus forming
hills in some cases not less than 250 feet in height. 1 In other in-
stances, as at Elbow Bay, on the south shore of the main island,
the sand, like a huge glacier, has quite filled a valley, and still
progressing in a mass some 25 feet in thickness j is covering
houses, gardens, and even woodlands, leaving, as at Lake Michi-
gan, only the trunks of dead trees standing partially exposed
in the midst of sandy plains.

One of the most interesting and remarkable of the many
regions for the observation of sand dunes, lies between Bor-
deaux and Bayonne in Gascony, and has been admirably de-
scribed by Reclus. 2 The sea here throws every year upon
the beach along a line 100 miles in length some 5,000,000
cubic yards of sand. The prevailing westerly winds, contin-
ually picking up the surface particles from the seaward side,
whirl them over to the inland or leeward slope, where they
are again deposited, and the entire ridge by this means alone
moves gradually inland. In the course of years there has
thus been formed a complex series of dunes all approximately
parallel with the coast and with one another, and of all alti-
tudes up to 250 feet. These are still marching steadily inward,
though at the rate of but 3 to 6 feet annually, and whole vil-
lages have more than once been torn down to prevent burial, and
rebuilt at a distance, to be again removed within 200 years. 3

The lithological nature of the dunes is widely variable, though
naturally siliceous sand is the prevailing constituent in the
majority of cases. J. W. Rutgers describes 4 the dune sands of
Holland as consisting principally of granules of quartz, to-
gether with those of garnets, augite, hornblende, tourmaline,
epidote, staurolite, rutile, zircon, magnetite, ilmenite, ortho-
el ase, calcite, and apatite ; and, more rarely, microcline, cor-
dierite, titanite, sillimanite, oliviiie, kyanite, corundum, and
spinel. The majority of these minerals occur in the form of
well-rounded granules, though many of the garnets, zircons,

Geology of Bermuda, Bull. 25, U. S. National Museum.

2 The Earth, Atmosphere, and Life.

8 The church of Lege, owing to the encroachment of the sand dunes, was
torn down in 1690, and rebuilt at a distance of 2$ miles from its first site.
By 1850 the dunes had traversed the intervening space, and again necessi-
tated its removal.

* Neues Jahrbuch fur Mineralogie u. Geologic, etc., 1895, 1 B., 1st Heft,

p. 22.

336 THE EEGOLITH

and magnetites show quite well-preserved crystal outlines. It
is noticeable that these sands contain no mica, although the
mineral occurs in the sea-sand, from whence the dunes are
derived. Rutgers accounts for this on the supposition that
during the transportation of the material the mica folia be-
come so finely shredded as to be sifted out from the heavier
particles of sand, and quite dissipated. It is well to note that
the abrasive power of wind-blown particles is greater than
that of those carried by water, since, as noted by Daubree, a
thin intervening film of water may serve to buoy up the gran-
ules, and keep them apart. To this fact is ascribed the angular
nature of many of the wind-blown grains. This same authority
seems to think that with wind-blown sand, as with water-worn
material, there is a minimum limit, beyond which reduction
in size of particles rarely goes. This minimum he places at
about .25 millimetre in diameter. It seems, however, more
probable that attrition may go on to an almost indefinite limit,
but that the finer and lighter materials are driven farther
away perhaps not collecting in the form of dunes at all
leaving, as one would naturally expect, the sands of any one series
of dunes of nearly uniform size. 1

It was noted by Blake during the surveys of the railway
routes to the Pacific that the wind-blown sands of the Colorado
desert were sometimes in the form of almost perfect spheres, all
their sharp edges and asperities having been worn away by
mutual attrition. The grains were composed mainly of quartz,
agate, garnet, and dark granules derived from the debris of vol-
canic rocks. In places there is a black iron sand, and usually
a considerable proportion of lime carbonate, as indicated by its
brisk effervescence when treated with acid. The sand dunes of
the Bermudas, as elsewhere noted, are composed wholly of cal-
careous material from finely comminuted shells and corals, while
those of the Sevier desert region of Utah, as described by Gilbert, 2
are of fine gypseous sand formed by the evaporation of the water
in the neighboring playa lakes.

Volcanic Dust. The finely comminuted materials ejected

1 Udclen has shown that the atmospheric currents being for the most part
loaded only to an insignificant fraction of their capacity, their sediments
will be more evenly assorted than those of water currents. (Journal of
Geology, Vol. II, 1894.)

2 Monograph I, U. S. Geol. Survey, 1890.

JEOLIAN DEPOSITS

337

from volcanoes and caught up by atmospheric currents, as de-
scribed on p. 122, are sometimes carried long distances to be
again deposited either on land or in the water, forming loose,
often flour-like deposits of varying thickness. At various points
in Colorado, Kansas, Nebraska, Montana, and other of the West-
ern states, are remnant beds of fine volcanic dust such as must
originally have covered many square miles of territory, the ma-
terials of which were de-
rived from sources now
wholly obscured. 1 The
illustration given on PL
28 is from a photograph,
taken by the writer, of
one of these beds in the
lower Gallatin valley,
Montana. From the
height of the man's
shoulder to his feet the
bed is of pure glassy dust,
very light gray in color,
and so fine and light that
when thrown into the air
it floats away at the slight- Fl(L 8 7.-Showing onttliii of shreds of vol-
est breath. r igure o7 canic dust, as seen under the microscope,
shows the appearance of

this glass as seen under the microscope. Beds of this nature up-
wards of 4 feet in thickness occur underlying the loess or surface
soil along the Republican River in Nebraska and Kansas and
even as far east as Omaha in the first-named state. The source
of their materials is problematical.

These aBolian deposits are of very recent origin, and the
beds loosely coherent. There are, however, good reasons for
supposing that similar processes were carried on in the earlier
stages of the earth 's history ; but that the peculiarly susceptible
deposits have since undergone such extensive alteration as
to be no longer recognizable as wind-drifted materials. Where
the material still exists as a surface deposit, it undergoes
ready decomposition on account of its porosity and easy permea-

1 See On Deposits of Volcanic Dust and Sand in Southwestern Nebraska,
Proc. U. S. National Museum, Vol. VIII, 1885, p. 99.
23

338

THE BEGOLITH

bility. The volcanic dusts are as a rule siliceous, more nearly
allied to the acid potash rocks than to the basalts.

The analyses given below show the chemical nature of (I) a
fine, white, almost flour-like pumice dust from Harlan County,
Nebraska, and (II) of dune sands from the Pamlico Peninsula,
North Carolina. This last is described 1 as a tolerably fine,
nearly white sand consisting of smooth, well-rounded grains,
mainly quartz, but containing also occasional shell fragments
and black granules of iron ore.

CHEMICAL ANALYSES OF VOLCANIC DUST AND DUNE SAND

CONSTITUENTS

Silica (SiO 2 )

69.12%

92.12%

Alumina (A1 2 3 ) |
Iron oxide (Fe 2 O 3 ) J '
Lime (CaO) . ....

17.64J
0.86

5.29
1.13

Magnesia (MgO) .

0.24

0.03

Potash (K 2 0)

6.64

0.64

Soda (Na 2 0)

1.69

0.35

Sulphuric acid (S0 2 )
ITiition ....

4.05

0.33
0.60

100.24 %

100.49%

(4) Glacial Deposits. Under this name are included those
drift deposits which are the product mainly of glacial action,
though their immediate deposition may have been brought about
in part through the instrumentality of water. The strictly
aqueo-glacial materials have been noted under the head of
alluvial deposits.

Allusion has been already made to the manner in which gla-
ciers erode and transport. During a comparatively recent
period in geologic history, there appears to have come over a
portion of North America a gradual lowering of the normal
temperature or increase in the annual precipitation, or perhaps
both, until the condition of affairs existing in northern Green-
land prevailed as far south as the 39th parallel of north lati-
tude. Now whether the ice sheet extended at any one time
over the area outlined below or whether there were periods
of advancement and retreat; whether the glaciation was pro-

1 Geology of North Carolina, Vol. I, 1875, pp. 182-183.

PLATE 29

FIG. 1. Section of glacial till.

FIG. 2. Glacial landscape.

GLACIAL DEPOSITS 339

duced by floating ice and local glaciers as argued by certain
Canadian geologists, or by a truly continental ice sheet thousands
of feet in thickness, are for our present purposes matters
of slight concern. We have more to do with results than
methods. Suffice it for the moment, that over the entire north-
eastern part of the United States and eastern Canada, all the ex-
isting loose materials from rock decay that had been gathering
for untold ages were carried bodily northward, westward, or
southward, as the case might be. From over a considerable part
of southern New England the original residual soils were stripped
and dumped into the Atlantic, portions of the transported mate-
rial still protruding above sea-level in the forms known now by
the names of Nantucket, No Man's Land, and Block Island. In
process of this transfer the rocks were planed down to hard
fresh surfaces, over and upon which were deposited new mate-
rials from the north. It follows that over this entire glaciated
area, estimated by Upham 1 as some 4,000,000 square miles, with
the exception of a few comparatively insignificant patches here
and there, scarcely a foot of clastic matter is to be found that
is truly native. Wherever road cuts or stream erosion favors,
the regolith in various conditions of compactness may be found
lying directly upon the hard, smooth, and striated rock with
which it has perhaps no affinity in composition or structure.
The rotten and mechanically triturated detritus of many rocks
from many sources more or less admixed by the moving glacier
or commingled by resultant streams, is spread out to form the
soils on land to which it is as truly foreign as are the emigrants
who land to-day upon our shores. The stone wall, built of
boulders found loose in the field, may consist of granites, dia-
bases, schists, or shales even though the underlying rock may be
a limestone; or the wall may be of limestone though the coun-
try rock be a gneiss, or slate. A similar distinction exists in
the soil itself, which, while it may in part consist of the material
of these boulders in a finely divided state, is more likely to con-
sist of detritus of softer rocks which yielded more readily to the
abrasive force. Sand and gravel or clay, dust or mud, black
with organic matter or red-brown from iron oxides, the ad-
mixture is ever varying, dependent only on the nature of the
materials to the north. But the material of the glacial drift
is spread out over the land in a manner far from uniform and
1 Ice Age in North America, p. 579.

340 THE EEGOLITH

under conditions widely variable. Following Professor Salis-
bury 1 and others, we may, according to its physical charac-
ters and method of deposition, separate the deposits into
two general groups: (1) the stratified or assorted drift, 2 and
(2) the unstratified or unassorted, the first having been laid
down under the influence of water and hence showing a more
or less stratified condition, while the second, deposited directly
from the ice, consists of a heterogeneous aggregate of coarse and
fine materials without evident marks of stratification. The two
forms are not always readily separable nor is their relative posi-
tion always the same, either one occurring uppermost, and "not
rarely they alternate with each other several times between the
surface and the bottom of the drift. ' '

A large part of the drift is composed of this unstratified and
unassorted material, consisting of clay, sand, gravel, and boulders
in ever-varying proportions, to which the name till or boulder
clay is commonly applied, or from its mode of deposition,
that of ground moraine. As already noted, it is the material
carried along beneath the ice sheet and left in the position it now
occupies on its final retreat. This, entirely unmodified except
upon the immediate surface where it has become converted into
soil through the agencies elsewhere described, forms the regolith
over large areas of the northeastern portion of America and of
northern Europe as well. Where as yet unaffected by oxidation,
it is of a gray or blue-gray color, and often so intensely tough and
hard as to necessitate, in process of excavation, recourse to blast-
ing. The upper portion, through percolation of meteoric waters,
is as a rule of a buff or brownish color, owing to oxidation of the
ferruginous constituents. Through the combined agencies of
this oxidation, of plant and animal life and of cultivation,
considerable contrasts in both physical and chemical properties
are brought about between the superficial and deeper-lying
portion, which are commonly recognized by the terms soil and
sub-soil respectively applied to them, though originally they
may have been one and the same thing. The composition of
this till naturally varies with the character of the rocks from
whence it was derived. It may have, and indeed probably has,
in most cases travelled but a short distance, and its constituent
particles may be the same as that of the rocks which it overlies,

4 Ann. Eep. State Geologists of New Jersey, 1891.

2 Here included in large part with the aqueo-glacial deposits.

GLACIAL DEPOSITS 341

though in a finely divided condition, only the harder and
tougher rocks retaining their lithological identity, the more
friable having been ground to the condition of clay and sand. 1
To attempt to give the composition of the till would necessitate
its study and analysis in innumerable localities, an endless and
profitless task. It will be sufficient to here describe a few repre-
sentative occurrences. In nearly all till the boulders, consisting
of the harder and more resistant of the materials, are in a more
or less rhomboidal form, with their surfaces scarred and with
other marks of the rough treatment to which they have been
subjected. They are in fact the tools with which the glacier has
done its work, and the scars are but the signs of wear. Inter-
mingled with these boulders is an ever-variable amount of finer
detritus, largely a result of mechanical abrasion. Professor W.
O. Crosby has studied in great detail the physical properties of
the till about Boston, and states 2 that, excluding the larger
stones, it consists of 25% of coarse material which may be classed
as gravel; 20% of sand; 40 to 45% of extremely fine sand, or
rock flour, and less than 12% of clay. The gravel in these cases
consists mainly of pebbles of the harder and more massive rocks
of the region, such as granite, diorite, diabase, quartzite, and
sandstone. In passing from gravel to sand, there is noted an
increase in the proportional amount of quartz, in clear and angu-
lar or subangular forms, due mainly to the disintegration of the
granite, quartzite and sandstone pebbles. The rock flour also
consists essentially of quartz. The most striking feature brought
out is the very small proportion of clay material, which varies
from one-tenth to one-eighth of the total bulk.

The table on the next page, as given by F. Leverett, shows the
approximate physical condition of the till as represented by the
sub-soil in various parts of Illinois.

The till is not, however, always spread out evenly over the
land, but though partaking in a general way of the topography
of the slopes which it covered, lies much deeper in certain

*Alden (Professional Papers, No. 24, U. S. Geol. Survey, 1904) found
that of the material in the glacial drifts of southeastern Wisconsin, from
three to thirty-two per cent, was foreign in the sense that the formation
whence it was derived did not occur within the area surveyed. Eighty-
seven per cent, of the drifts, as a whole he regarded as local, the remaining
thirteen per cent, having come from distances as remote as one hundred
miles.

2 Proc. Boston Soc. of Natural History, 1890, p. 123.

342

THE EEGOLITH

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GLACIAL DEPOSITS 343

places than others. Indeed, it thickens and thins out very
irregularly and in many places fails entirely either through
having never been deposited, as over many a rocky hillside in
New England, or through having been removed by running
water. Moreover, there are found in certain parts of the drift-
covered areas rounded hills of very symmetrical form, composed
of material identical with the till, but which must have been
deposited under slightly different conditions. These range in
height up to 200 or 300 feet, though rarely more than half that
amount. Such forms are known as drumlins.

The terminal moraines represent those portions of the drift
which gathered near the edge of the ice sheet in the form of
submarginal accumulations, to be left as broad belts or ridges
of sand and gravel on its retreat. Such with reference to
their position to the margin of the ice are known also as
terminal, marginal, or frontal moraines. The materials of which
they are composed represent (1) that which accumulated be-
neath the edge of the ice while it was practically stationary for
a considerable length of time; (2) that dumped from the
surface at its margin ; and (3) that pushed up by the ice sheet,
in front of itself during its forward movement. Such ridges
are not sharp as a rule, but broad and low, it may be from a
fraction of one to several miles in width. Unlike the subgla-
cial drift, the till, the materials are but loosely consolidated,
and but a small part, if any, of the boulders show the scarred
and abraded surfaces so characteristic of those of the till proper.

The frontal moraine, occupying the southern and western
margin of the glaciated area, forms one of the most striking
and unique geological bodies. Composed of materials of a
most heterogeneous nature, ever varying, and limited in range
of variation only by the lithological character of the rocks to
the northward and eastward; in all degrees of coarseness and
fineness, from boulders of many tons' weight to particles too
small to be visible to the unaided eye, only obscurely and some-
times scarcely at all stratified excepting where subsequently
modified by running water; in the form of broad low hillocks,
domes, and ridges, the moraine sweeps in an interrupted, sin-
uous belt from eastern Massachusetts to North Dakota and over
400 miles into British America, having a length, in all its wirici-
ings and turnings, of not less than 3000 miles.

The water arising from the melting ice sheet flowed off, in

344 THE KEGOLITH

part, over the surface, forming superglacial streams, or in part
upon the surface of the ground beneath as subglacial streams,
of which last the river Rhone of to-day is a good example.
Presumably also a portion of the water became concentrated
and flowed for short distances in the mass of the ice itself,
forming thus englacial streams. In all cases the running water
would collect, reassert, and variously modify the rock debris
found either in immediate connection with the ice itself or at
its extremity, in the terminal moraines. There were thus
formed hillocks and ridges or low fan-shaped masses of "modi-
fied drift." The sand, gravel, and boulders which collected in
the troughs of superglacial streams would, on the final melting
of the ice, be deposited as ridges running essentially parallel
with that of the movement of the ice on which they formed.
Such are known as eskers, or osars. Other deposits closely
resembling these and sometimes confounded with them, but
formed, it is believed, only by swift and changeable currents
near the frontal margin of the ice, present often a rude and
disturbed and distorted stratification, and are known as kames.
They differ from the eskers in their outlines as well as positions
with reference to the glacier from whence their materials were
derived, being as a rule in the form of hills, rather than ridges,
and with their longer axes at right angles with that of the ice
motion.

Beyond the margin of the ice and its terminal moraines are
found still other loosely aggregated deposits of a similar hetero-
geneous nature which are likewise due to swiftly running water
caused by the melting ice. Such, according to their position
and form, are known as valley drift, morainic or frontal aprons,
and overwash plains.

The thickness of these glacial deposits varies greatly, as has
been already indicated. Variations of upwards of a hundred
feet may occur within the limits of even less than one square
mile. Professor Newberry estimated that the area south and
west of the Canadian highlands covered with glacial drift was
not less than 1,000,000 square miles, and that its average
depth would not be less than 30 feet. Other estimates on
deposits in Ohio, Indiana, and Illinois give an average thickness
in these states of 62 feet. In extreme cases the deposit has
been found to extend to a depth of 300 to 500 feet. Bell has

THE SOIL 345

stated 1 that glaciation of the surface of British America has
been almost universal in the regions east of the Rocky Moun-
tains, and all over the Palaeozoic districts west and south of
Hudson and James Bay the average depth of the till is 100 feet,
and perhaps 200 feet in Manitoba and the northwest territories.
The following section is given by James Geikie 2 as showing
the varying character of the glacial drift and its interstratified
interglacial lacustrine deposits:

FEET INCHES

Sandy clay 5

Brown clay and stones (till) .... 17

Mud 15

Sandy mud 31

Sand and gravel . . 28

Sandy clay and gravel 17

Sand 5

Mud 6

Sand 14

Gravel 30

Brown sandy clay and stones (till) . . 30

Hard red gravel 4 6

Light mud and sand . 1 8

Light clay and stones 6 6

Light clay and whin block 26

Fine sandy mud 36

Brown clay, gravel, and stones ... 14 4

Dark clay and stones (till) 68

355
3. THE SOIL

There remains now to be summarized a few of the character-
istics of those superficial portions of the regolith to which the
name soil is commonly applied, and these, too, only in direct
relation to their properties as soils, since as integral portions
of the regolith they have already been sufficiently touched upon.

(1) The Chemical Nature of Soils. The prevailing con-
stituents of any soil, whatever its source, is nearly always silica,
with varying amounts of alumina, oxides of iron, lime, magnesia,
and the alkalies. 3 A small amount of organic matter, from
extraneous source, is usually present. This prevalence of silica
and alumina as may be readily understood, is an essential conse-

1 Bull. Geol. Soc. of America, Vol. I, 1890, p. 289.

2 The Great Ice Age, 3d ed., 1894, p. 120.

* The peat deposits furnish almost the only exception to this rule.

346 THE REGOLITH

quence of soil formation through the breaking down of rocks
by the processes of weathering, whereby all but the most in-
destructible portions are lost.

The predominantly inorganic nature of any soil may easily
be shown by fractional separations, made either by washing,
or by sieves of varying degrees of fineness, whereby it is
brought into portions of like size and weight such as can con-
veniently be submitted to microscopical and chemical analyses.
All portions, from the finest dust to particles of such size as to
be classed as pebbles, will thus be found to be but mineral
matter, particles of quartz, feldspar, shreds of mica, and other
silicates in ever-varying proportions and stages of alteration
or decomposition.

Owing to the destructive nature of their formation, it is but
natural that a soil, particularly one of considerable antiquity,
should but slightly resemble the parent rock. This fact was
more than suggested in the chapter on rock-weathering. In
order that its significance may be fully comprehended, the
analyses of fresh rock and corresponding residual material from
various sources are given in the table on the next page.

The most striking of the dissimilarities shown by this table
are, as is to be expected, those of the limestone soils, in columns
I and II, where the proportional amounts of silica, iron and
alumina are increased, roughly speaking, nearly one hundred
fold, while the amount of lime carbonate is correspondingly
diminished. This condition of affairs is still further exag-
gerated in the case of the red soil of Bermuda (columns III and
IV) which offers particularly favorable opportunities for study,
owing to the isolated condition of the islands and the consequent
freedom from danger of contamination by other than local drift.

The shells and corals which in a more or less consolidated con-
dition form the entire mass of these islands, although essentially
of carbonate of lime, are nevertheless not entirely so, carrying,
aside from the magnesia, about 1% of inorganic impurities,
chiefly oxides of iron and alumina and earthy phosphates, which
are practically insoluble in the water of rainfalls, with which
alone we have to do here. As time goes on, the lime is slowly
leached out and carried away into the ocean, the insoluble parts
remaining. Throughout the centuries of decay, this 1% of
insoluble impurities, representing but one ton of residue to
every 99 tons removed, slowly accumulates until it forms the

THE SOIL

347

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348 THE REGOLITH

common red earth of the islands. Though usually fertile, where
the leaching has been excessive the resultant soil is so rich in
iron and other deleterious constituents as to be quite barren.

There are few more impressive facts in agricultural geology,
than that each foot in depth of such soil, as it now lies at our
feet, may indicate the removal of at least 100 feet in actual thick-
ness of limestone. In other words, even assuming that nothing
has been lost by mechanical erosion, the surface of the ground
has been lowered this much in bringing about the present con-
ditions.

From what has gone before, it is obvious that soils derived
by purely mechanical agencies will, if unmixed with other ma-
terials, show a composition closely resembling the mother rock,
as in the case of that derived from granite as described on p. 186
or those derived from argillites and siliceous sandstones; others
in which chemical agencies prevailed may by solution and other
changes have so far lost important constituents as to be scarce
recognizable as rock derivatives at all. Obviously a rock mass
containing in itself none of the elements of plant food cannot,
merely through its decay, furnish soil of appreciable fertility.
This fact is well illustrated in the region known as the Bare
Hills north of Baltimore, Maryland, or the Chester County
Barrens in southern Pennsylvania. Both regions are under-
laid by peridotites rocks rich in iron-magnesian silicates, but
almost wholly lacking in lime, potash, or other desirable con-
stituents. Such rocks not merely decompose very slowly, but
the stingy product of their decomposition consists only of hya-
line forms of silica, magnesian carbonates, or silicates and fer-
ruginous products quite devoid of nutrient matter, affording
food and foothold to scanty growths of grass and stunted
shrubs. That, however, a rock contains all the desired mate-
rials, is no certain indication as to character of its decomposition
product, since in the process of decomposition much desirable
matter may have become lost. Nevertheless most soils retain
what we may call inherited characteristics, and a direct com-
parison whenever possible is by no means uninteresting, as will
be noted later.

It need scarcely be remarked that the value of any soil de-
pends wholly upon its capacity for plant growth. Hence a
satisfactory treatise on the subject should be written with a
view to showing to what this capacity is due, and what are

CHEMICAL NATUKE OF SOILS 349

the laws governing its fertility and its rejuvenation when that
fertility becomes exhausted. Such a method of treatment is,
however, far beyond the limits of the present work, and we must
content ourselves with merely touching upon a few of the most
salient points, leaving the at present little understood subject
of fertility for other and abler writers. It may be well to re-
mark, however, that a soil left to itself and nature's processes
rarely becomes barren or exhausted except it may be under
changed geological conditions. A growing organism takes
temporarily from the soil that which is essential, but restores
it again with accrued interest in the form of carbonaceous and
nitrogenous matter derived from the atmosphere, when it dies.
Thus, under normal conditions, the soil grows yearly richer
and richer and capable of supporting larger and more luxuriant
crops. It is only when the husbandman comes in, and by his
improvident harvesting robs the soil not merely of its interest
due, but of a part of the principal as well, that bankruptcy
results.

For a long period the fertility of a soil was felt to be dependent
very largely upon its chemical composition, and older treatises
and reports of geological surveys are filled with tables of analy-
ses which the acquired knowledge of years now shows us to be
almost worthless, either for the purposes for which they were
first intended, or as indicative of the mineral nature of the soil
itself. 1 A soil which, under certain conditions of climate or
moisture, is utterly barren may, under changed conditions, be
fruitful in the extreme, as has been repeatedly demonstrated in
the case of the so-called American deserts, dreary stretches of
aridity given over to sage brush and a few degraded forms of
animal life, but which need only moisture to cause them to
laugh with harvests.

Naturally, a soil containing in itself nothing in the way of
available plant food can be made to produce crops only when

1 The common practice of making soil analyses, whereby the results are
tabulated as soluble and insoluble (meaning by soluble the portion extracted
by boiling hydrochloric acid) and putting down the latter as silica (or
sand) and insoluble silicates, cannot be too strongly condemned. It means
nothing. A growing plant is capable of extracting only a small, and as
yet unknown, portion of that taken out by the acid, and as to what silica
and insoluble silicates may be, we are left in ignorance. Such analyses are
satisfactory to neither the student of soils nor of geology. When quoted in
this work it is merely because nothing better is available.

350 THE EEGOLITH

the needed constituents are supplied. Investigations have,
however, shown that, though varying in different species, the
proportional amount of food demanded by plants which can be
supplied by the atmosphere and meteoric waters is very large.

It seems to be now pretty well conceded that of all the con-
stituents found in soil aside from moisture, only potash, lime,
magnesia, phosphoric and sulphuric acids, can be considered
absolutely essential as plant food. The ash of all plants, to be
sure, contains silica, soda, and it may be iron and other min-
eral ingredients, but such are to be regarded as accidental
rather than otherwise. Of the constituents enumerated as
essential, magnesia and sulphuric acid are almost invariably
present in sufficient quantities, while potash, lime, and phos-
phoric acid, even though sufficiently abundant in a virgin soil,
are liable to exhaustion under the ordinary methods of culti-
vation. The source of these materials has been shown in the
previous pages and need here be only touched upon. The
potash and the lime must have come originally from the de-
composition of potash-lime-bearing silicates, as the feldspars and
micas, amphiboles and pyroxenes. The original source of the
phosphoric acid was undoubtedly the apatite of the eruptive
rocks, though now to be found in bones and skeletons of ani-
mals, whose remains become entombed in sedimentary rocks
of all ages. How small and proportionally insignificant are
the percentages of these constituents in any soil, fertile or
barren, is shown in the table on p. 351, 1 in which are given the
general average composition of a large number of soils, seden-
tary and transported. The sulphuric acid, which is not men-
tioned in this table, rarely amounts to more than from 0.05%
to 0.5% when calculated as sulphuric anhydride (S0 3 ).

So small, comparatively, are these percentages, that it is rare,
indeed, to find a soil which on complete analysis will not be
shown to contain them in sufficient quantity. The varying
degrees of fertility in such cases are due then, not to differ-
ences in ultimate composition, but to difference in combination
of these elements whereby they are or are not available for
plant food, and to physical and climatic differences as well.
Naturally a growing plant can take up only that which is
soluble by the means at its command. A high percentage of

1 rrom Part A, Vol. II, Part II, Chemical Analyses, Geological Survey
of Kentucky, p. 113.

CHEMICAL NATUKE OF SOILS

351

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352 THE EEGOLITH

any of the above constituents counts for little when they are
combined in the form of difficultly soluble silicates. A granitic
rock, as has already been noted, contains locked up in its mass
all the mineral elements necessary for a fertile soil, but remains
barren simply because these are in a condition of slight solu-
bility and its physical structure is such that even the soluble
portions are unavailable. Pulverize this rock sufficiently, and
it will become immediately available for soil, though naturally
its fertility is slight, and rendered enduring only by gradual
decomposition. It is of course possible, that by nature 's methods,
decomposition and incident leaching may have gone so far that
a soil on the immediate surface, though derived from rocks rich
in essential constituents, has become quite impoverished and
barren. This is especially true with limestone residuals, as has
been already noted. It is doubtless to this fact that is due the
enduring qualities of the glacial till as a soil, though its immedi-
ate fertility may not be as great as one of sedentary origin. The
undecomposed feldspathic and other mineral particles contained
by the till, due to its mechanical origin, yield up slowly but
continually their supply of plant food, and such a soil may long
outlast the residual clays of non-glaciated regions.

Soils derived from deposits of modified glacial drift are
almost invariably sandy or gravelly in their nature. Such, on
account of their easy working qualities, great porosity, and
ready permeability, are commonly known as light soils, even
though their actual specific gravities may be greater than the
so-called heavy soils of the ground moraine. 1

1 Mechanical analysis of a glacial soil from an old pasture, Cape Eliza-
beth, Maine, yielded results as below. The portion selected was of just the
thickness turned up by the plough, about 7 inches. In color it was dark
gray, at the immediate surface almost black from organic matter, and
penetrated throughout by grass roots. Fine angular grains of white quartz
were the most conspicuous feature on macroscopic examination. Eight hun-
dred and thirty grammes of this soil on sifting yielded: (1) 2.5 grammes
gravel, which failed to pass a sieve containing 8 meshes to the lineal inch.
This consisted mainly of angular quartz and cleavage bits of feldspar with
occasional rounded lumps of impure limonite, and not completely disin-
tegrated particles of granitic rock. (2) 40 grammes coarse sand retained
by 20-mesh sieve and consisting of clear glassy and white opaque quartz
in angular and sub-angular fragments, the largest forms being some 3
millimetres in greatest diameter; cleavage bits of white and pink feldspar,
rarely folia of white mica, a few bits of mica schist, and lastly hard,
rounded pellets of indurated silt and organic matter. (3) 170 grammes

CHEMICAL NATUKE OF SOILS 353

There is many an humble homestead throughout the glaciated
areas of North America whose lack of worldly prosperity is due
to the dry and barren soil supplied by these deposits of modi-
fied drift. On the other hand, there are numerous regions, like
those of northern Ohio, where a light, barren, residual soil de-
rived from sandstone has become enriched by an admixture of
glacial clays from the north, and thus brought prosperity to
thousands of happy homes. Nature works out her own com-
pensations, impoverishing, it may be, here but correspondingly
enriching there.

retained by 40-mesh sieve and consisting of a clean sand composed of some
two-thirds its bulk white quartz particles and one-third opaque, partially
kaolinized feldspathic particles; rarely any mica or free iron oxides. (4)
180 grammes retained by 60-mesh sieve and consisting, like the last, of
clean quartz and feldspar sand, the quartz particles in excess of the feld-
spar, and rarely a little mica. (5) 82 grammes retained by the 80-mesh
sieve. This, very clean sand of quartz and feldspar, in the proportion of
about three fifths quartz and two fifths feldspar. (6) 150 grammes retained
by a sieve of silk bolting cloth of 120 meshes to the lineal inch. Like the
last, composed almost wholly of bright quartzes and somewhat kaolinized
feldspars with scarcely a trace of other silicates. (7) 185 grammes which
passed the silk bolting cloth. This was submitted to washing, the lighter
finer material being poured off as silt. By this means were obtained 118
grammes very fine sand and 67 grammes silt. The fine sand, as before,
showed under the microscope only quartz and feldspars, the quartzes still
in excess. The silt to the naked eye consisted of a light brown, almost
impalpable material, which the microscope resolved into quartz and feldspar
particles with shreds of ferruginous products evidently derived from the
decomposition of iron-magnesian silicates, such as micas or amphiboles.
(8) Organic matter, 19.5 grammes.

A bulk analysis of the air dry-soil, excluding all grass and roots, yielded
results as below:

Ignition (water and organic matter) .... 2.72%

Silica 76.80

Alumina and iron oxides 14.04

Lime 0.78

Magnesia Traces

Potash 2.87

Soda 1.18

98.39%

Such a soil is plainly little more than a highly quartzose granite or gneiss
in a pulverulent condition and in which the agencies of decomposition have
scarcely begun their work. Its composition could have been almost foretold
by the microscopic examination.
24

354

THE KEGOLITH

E. H. Loughbridge has shown 1 that the percentage of soluble
material in a soil rapidly increases with the degree of commi-
nution; i. e., the finer the material the larger the proportional
amount of soluble matter, and hence of matter available as
plant food. This is well brought out in the following table
abridged from the one given in Mr. Loughbridge 's original
paper, the figures in the upper space of each column indicating
the size of the particles, and the percentage amount of each as
determined by fractional separations.

PERCENTAGES OF SOLUBLE MATTER IN SOILS

CONVENTIONAL NAME :

CLAY

FINEST SILT

FINE SILT

MEDIUM

SILT

COARSE
SILT

DIAMETER OP PARTICLES :

21.64%

23.56%
mm.

.005-. Oil

12.54%
mm.
.013-.016

13.67%
mm.

.022-.027

13.11%
mm.
.033-.038

CONSTITUENTS

Insoluble residue ....
Soluble silica . ...

15.96
33.10

73.17
9 95

87.96
4 27

94.13
2.35

96.52

Potash (K 2 O)
Soda (Na 2 0)
Lime (CaO) . .

1.47
1.70 2
009

0.53
0.24
13

0.29
0.28
18

0.12
0.21
09

....

Magnesia (MgO) ....
Manganese (Mn0 2 ) . . .
Iron sesquioxide (FegOg) .
Alumina (A1 2 O 3 ) ....
Phosphoric acid (P 2 O 5 ) . .
Sulphuric acid (SO 3 ) . . .
Volatile matter ....

1.33
0.30
18.76
18.19
0.18
0.06
9.00

0.46
0.00
4.76
4.32
0.11
0.02
5.61

0.26
0.00
2.34
2.64
0.03
0.03
1.72

0.10
0.00
1.03
1.21
0.02
0.03
0.92

....

Totals

100 14

99 30

100 00

100 21

96 52

Total soluble constituents .

75.18

20.52

10.32

5.16

That the soluble constituents are, however, more available in
these more finely comminuted soils is perhaps an open question,
since, as pointed out by Van Hise, 3 the rapid solution of the finer
particles could very likely be more than counterbalanced by
the slower circulation of the underground waters.

1 On the Distribution of Soil Ingredients among the Sediments obtained
in Silt Analysis, Am. Jour, of Science, Vol. VII, 1874, p. 17.

2 An excess of original amount, due to the addition of sodium chloride to
produce flocculation of clay in suspension.

3 Treatise on Metamorphism, p. 155.

CHEMICAL NATUKE OF SOILS 355

According to Hilgard, 1 the substance which assumes com-
manding importance as controlling the fertility of a soil, aside
from physical conditions, is lime, in the presence of which, in
adequate proportions, smaller percentages of the other plant
foods will suffice for high and lasting productiveness, than
would otherwise be the case. Since lime is the essential con-
stituent of the rock limestone, it follows that, other things
being equal, a "limestone country is a rich country." As else-
where noted, however, a limestone soil may have become so
leached of its lime, through prolonged decay, as to be benefited
by artificial applications of this same constituent. Lime is,
moreover, so generally distributed throughout the great majority
of rocks that few soils would be lacking in this constituent
were even a small proportion of the original amount left in the
residue from rock decay, instead of being so largely removed
in solution.

It would follow from this that the composition and fertility
of a soil is dependent not more upon the character of the rock
mass from which it is derived, than upon the prevalent climatic
conditions under which it originated, the general average tem-
perature and the amount and distribution of the rainfall being
particularly important factors. This branch of the subject has
also been considered in some detail by Hilgard, to whom we are
indebted for the only satisfactory resume. Concerning condi-
tions of temperature, this author says:

"Within the ordinary limits of atmospheric temperatures all
the chemical processes active in soil formation are intensified
by high and retarded by low temperatures, all other conditions
being equal. This being true, we would expect that the soils
of tropical regions should, broadly speaking, be more highly
decomposed than those of the temperate and frigid zones.
While this fact has not been actually verified by the direct
comparative chemical examination of corresponding soils from
the several regions, yet the incomparable luxuriance of the
natural as well as the artificial vegetation in the tropics, and
the long duration of productiveness, offer at least presumptive
evidence of the practical correctness of this deduction. In
other words, the fallowing action, which in temperate regions
takes place with comparative slowness, necessitating the early

i The Relation of Soil to Climate, Bull. No. 3, U. S. Weather Bureau,
1892.

356 THE BEGOLITH

use of fertilizers on an extensive scale, has been much more
rapid and effective in the hot climates of the equatorial belts,
thus rendering available so large a proportion of the soil's in-
trinsic stores of plant food that the need of artificial fertilization
is there restricted to those soils of which the parent rocks were
exceptionally deficient in the mineral ingredients of special
importance to plants that ordinarily form the essential material
of fertilizers." 1

Concerning the concentration and leaching out of certain con-
stituents by the action of meteoric waters, the same authority
says:

' ' When, however, the rainfall is either in total quantity or in
its distribution insufficient to effect this leaching, the sub-
stances which otherwise would have passed into the sea are
wholly or partially retained in the soil stratum, and when in
sufficient amount may become apparent on the surface in the
form of efflorescences of 'alkali' salts. One of the most im-
portant modifications produced by scantiness of rainfall on soil
formation is the great retardation of formation of clay from
feldspathic rocks (kaolinization) and the sediments derived
therefrom. As a result, it is observed that the soils of the
Atlantic slope are prevalently loams, containing considerable
clay, and even in the case of alluvial lands, oftentimes very
heavy, while the character of the soils of arid regions is pre-
dominantly sandy or silty with but a small proportion of clay,
unless derived directly or indirectly from clay or clay shales.
In the former case, the clay, becoming partially diffused in
the rain water when a somewhat heavy fall occurs, percolates
through the soil in that condition and tends to accumulate in
the sub-soil, the result being that almost without exception,
the sub-soils of the humid regions are very decidedly more
clayey than the corresponding surface soils. Not only does
this clay water tend to make the sub-soil more compact and
heavy, making it less pervious to water and air, but it is as-
sisted materially in this by the action which tends to leach the

1 While the action of frost in bringing rock masses into the condition of
soil is, in temperate climates, of very great importance, there seems to be
a limit beyond which it accomplishes little in the way of directly promoting
decomposition, and presumably disintegration as well. Collier's (8th Ann.
Eep. New York Exp. Station, 1889) experiments showed that 47 successive
freezings and thawings of a soil did not perceptibly increase the percentage
of soluble potash.

CHEMICAL NATUKE OF SOILS

357

lime carbonate out of the surface soil into the sub-soil. The
accumulated clay is thus frequently more or less cemented into
a 'hardpan' by lime partly in the form of carbonate and partly
in that of zeolitic (hydrous silicate) compounds, adding to the
compactness of the sub-soil, and therefore to the usual specific
difference between the soil and sub-soil; viz., the deficiency or
absence of humus and the difficulty of penetration by an aera-
tion of the roots of plants."

For these reasons the soils of arid regions, even though con-
taining the same materials, are often of uniform physical and
chemical character to great depths. The soluble salts, as car-
bonate of lime and salts of potash and soda, which are leached
away in regions of great average humidity, remain in those
where the annual precipitation is less, or where, on account of
its uneven distribution throughout the warmer months of the
year, its permeability and consequent leaching action is less.
Hilgard brings out this fact prominently in tables from which
that below is condensed, the original being compiled from sev-
eral hundred analyses of soils from the humid regions of North
and South Carolina, Georgia, Florida, Alabama, Mississippi,
Arkansas, Kentucky, and the arid regions of California, Wash-
ington, Montana, Utah, Colorado, Wyoming, and New Mexico.

SHOWING THE PROPORTIONAL AMOUNTS OF SOLUBLE SALTS IN SOILS OP ARID
AND HUMID EEGIONS

CONSTITUENTS

ARID REGION

HUMID REGION

Insoluble residuo

69.681 %

84.472 %

Solubl6 silica

6.289

3.873

Potash

0.825

0.187

Soda

0.251

0.071

Lime

1.645

0.112

Magnesia

1.384

0.209

Brown manganese oxide . ....

0.056

0.126

Iron peroxide

5.431

3.455

Alumina

7.309

4.008

0.14"4

0.114

Sulphuric acid

0.035

0.065

Water and organic matter

5.585

3.557

Total .

98.635%

100.149%

Discussing these figures, Professor Hilgard says: "Concern-

358 THE EEGOLITH

ing this table with reference to the lime, a glance at the col-
umns for the two regions shows a surprising and evidently
intrinsic and material difference approximating to the propor-
tion of 1 to 14J. This difference is so great that no accidental
errors in the selection of analysis of the soils can to any mate-
rial degree weaken the overwhelming proof of the correctness
of the inference drawn upon theoretical grounds; viz., that the
soils of the arid regions must be richer in lime than those of
the humid countries." These remarks hold good also for the
percentages of magnesia and the alkalies. From the fact that
in humid regions the more soluble constituents are leached out,
we may safely infer a corresponding proportional increase in
the insoluble constituents. This is also made manifest by the
tables, there being a difference of nearly 15% in favor of the
humid regions. The table shows, further, a probably greater
proportion of "zeolitic" material in the soil of arid regions, the
assumption being based upon the percentages of soluble silica.
Concerning this difference, the author says:

"Nor should this be a matter of surprise when we consider
the agencies which -are brought to bear upon the soils of the
arid regions with so much greater intensity than can be the
case where the solutions resulting from the weathering process
are continually removed as fast as formed by the continuous
leaching effect of atmospheric waters. In the soils of regions
where summer rains are insignificant or wanting, these solu-
tions not only remain, but are concentrated by evaporation to
a point that in the nature of the case can never be reached in
humid climates. Prominent among these soluble ingredients
are the silicates and carbonates of the two alkalies, potash and
soda. The former, when filtered through a soil containing the
carbonates of lime and magnesia, will soon be transformed into
complex silicates in which potash takes the precedence of soda,
and which, existing in a very finely divided (at the outset in a
gelatinous) condition, serve as an ever-ready reservoir to catch
and store the lingering alkalies as they are set free from the
rocks, whether in the form of soluble silicates or carbonates. 1
The latter have still another important effect. In the concen-
trated form, at least, they themselves are effective in decom-
posing silicate minerals refractory to milder agencies, such as
calcic carbonate solutions, and thus the more decomposed state

1 See author 's remarks on page 363.

CHEMICAL NATURE OF SOILS 359

in which we find the soil minerals of the arid regions is intel-
ligible on that ground alone. But it must not be forgotten
that lime carbonate, though less effective than the corresponding
alkali solutions, nevertheless is known to produce, by long-
continued action, chemical effects similar to those that are more
quickly and energetically brought about by the action of
caustic lime. In the analysis of silicates we employ caustic
lime for the setting free of the alkalies and the formation of
easily decomposable silicates by igniting the mixture; but the
carbonate will slowly produce a similar change, both in the
laboratory and in the soils, in which it is constantly present.
This is strikingly seen when we contrast the analyses of calca-
reous clay soils of the humid region with the corresponding
non-calcareous ones of the same. In the former the propor-
tions of dissolved silica and alumina are almost invariably much
greater than in the latter so far as such comparisons are prac-
ticable without assured absolute identity of materials. "

It is evident from the above that, provided the amount of de-
composition be the same, the soil of an arid region may contain
a larger proportion of desirable constituents than one in a region
of considerable annual precipitation. It may, also, and for the
same -reasons, contain a larger proportion of constituents that
are positively deleterious. This is particularly true of arid and
semi-arid regions of poor drainage, like the Great Basin regions
of the United States, where salts of sodium accumulate to such an
extent as to render the land sterile and barren in the extreme.

The primary origin of the sodium in these salts lies in the
soda-bearing silicate minerals forming the rocks of the region
and from which they have been set free through their decom-
position.

It should be stated, however, that the so-called "alkali" is
not composed wholly of sodium compounds, but contains also
salts of magnesia, lime, iron and potash. Nor is the form under
which the salts exist at all constant. As a rule, the larger por-
tion of the alkali is in the form of sulphate of soda, though a
considerable portion may exist as carbonate or chloride, and
smaller proportions in the form of nitrates. Concerning the
formation of these carbonates, Hilgard says: 1

"There seems to be a consensus of opinion that the carbona-
tion of the soda is connected in some way with the presence

1 Bull. No. 3, Weather Bureau, IT. S. Dept. of Agriculture, 1892.

360 THE EEGOLITH

of limestone or carbonate of lime, and that an exchange has
occurred in which either common salt or Glauber salt have trans-
ferred their acidic components to lime and have become car-
bonates instead. . . . Yet the simple explanation of the con-
trary reaction was given and published as early as 1826 by
Schweigger. In 1859 it was again observed by Alex Muller,
in a different form, but neither of these chemists, nor any of
their readers, appear to have perceived the important bearing of
this reaction, not only upon the formation of the natural depos-
its of carbonate of soda, but also upon a multitude of processes
in chemical geology. Without going into details ... it may
be broadly stated that the formation of carbonated alkalies oc-
curs whenever the neutral alkaline salts (chlorides or sulphates)
are placed in presence of lime or magnesia carbonates and car-
bonic acid, or of alkali ' supercarbonates ' (hydrocarbonates) con-
taining even a slight excess of carbonic acid above the normal
carbonates, the latter being the actual condition of all natural
sodas. ' ' 1

We have thus far considered only those elements of the soil
that are derived directly from the rocks from which they are
formed.

To this list we should add the element nitrogen, not so -much
on account of its quantity, as its value as plant food and of the
great economic value of some of its compounds. The common
forms under which this element exists, are (1) atmospheric
nitrogen, a colorless, tasteless, and innocuous gas which forms
some three-fourths by bulk of the air we breathe, and (2) the
nitrogen of the soil, where it exists in at least three distinct
forms, (1) organic nitrogen, (2) as ammonia or ammonia salts,
and (3) as nitric acid.

The average amount of nitrogen present in agriculture soils
is given by authorities as varying from 0.1% to 0.3%, though
occasionally, as in certain soils rich in organic matter, 4 or 5%.
Of these forms only the ammonia salts and nitric acid are of
direct value for plant food. Nitrogen, in the form of nitrate
of soda, forms an important mineral fertilizer, as noted on p. 67.

The extraordinary richness in nitrates of the soils in tropical
countries, and particularly in South America, has often been

1 See further the Mineral Constituents of the Soil Solution, by F. K.
Cameron and J. M. Bell, Bull. 30, Bureau of Soils, U S. Dept. of Agricul-
ture, 1905.

MINERAL NATURE OF SOILS

361

remarked since the subject was first broached by Humboldt
and Boussingault. According to Muntz and Maracano, nitrates
occur in the soils of Venezuela, the valley of the Orinoco, and
other localities sometimes to the amount of 30% of their mass.
These nitrates they show to be due to the oxidation of organic
nitrogen through the agency of bacteria. They state that in
the caverns of the regions, a guano composed mainly of the
excreta of birds and bats, but admixed also with the dead bodies
of these and other animals, has accumulated to the amount of
millions of cubic metres. Through the gradual nitrification of
this guano, and a combination of the nitrogen with the lime
of bones, or existing as a carbonate in the soil, a gradual transi-
tion is brought about wherever there is free access of air or
the temperature is sufficiently high to stimulate the nitrifying
organisms to their fullest activity. There is thus a gradual
change in the character of the nitrogeneous combination from
the interior to the exterior portions of the cave, as shown in the
following:

ANALYSIS OF BAT GUANO

CONSTITUENTS

GUANO FROM
INTERIOR OP
CAVE

EARTH FROM
THE ENTRANCE

EARTH FROM
SOME DISTANCE
FROM ENTRANCE

Organic nitrogen
Nitrate of lime

H.74%
0.00

2.41 %
3.03

0.80o/
10 36

These authorities would account for the presence of extensive
deposits of nitrates as in Chili, on the assumption that the solu-

NlTROGEN AND NITRATES IN SOILS

CONSTITUENTS

SAN JUAN

Los MORROS
DE PARAPARA

EL ENCANTADO

Nitrate of iime
Organic nitrogen

2.85 %
0.15

3.50%
0.27

0.62 %
0.21

ble nitrate, originally derived from the decomposing organic mat-
ter, as noted above, had been leached out from its place of origin
by percolating water and redeposited elsewhere on evaporation.

362 THE KEGOLITH

The invocation of atmospheric electricity to account for any
part of the nitrates of the soils, they regard as quite unneces-
sary, the same being of indirect influence only, furnishing first
nitrogen for growing plants which in their turn serve as food
for animals. These same authorities give the figures shown in
table at bottom of page 361 relative to South American soils.

(2) The Mineral Composition of Soils. This is essentially
the same as that of the regolith of which the soil forms a part.
Fragmental quartzes and feldspars form the larger proportion
of most soils. These are intermingled with shreds of mica,
amphibole, pyroxene, calcite or aragonite, iron and manganese
oxides, and in variable proportions, kaolin and other silicates,
carbonates and oxides. The presence of these constituents is
usually somewhat obscured by iron oxides and carbonaceous
matter; but when these are removed by acids or by ignition,
and the residue submitted to microscopic analyses, the true
mineral nature can be, as a rule, made out with approximate
accuracy. 1

From what has gone before, it must be evident that the con-
stituents of any soil are almost universally in a finely fragmen-
tal condition, and, with the exception of quartz and the rarer
minerals, as tourmaline, garnet, zircon, etc., in varying and
often advanced stages of hydration and decay. 2 Silica in the
form of free quartz and various silicates, alumina as hydrous
silicates, and iron as hydrated oxides, make up from 80% to
90% of the superficial portions of most deposits of this nature.
It is possible that under favorable conditions new minerals
may be temporarily formed. Alumina in the form of hydrated
oxides diaspore, beauxite, gibbsite, hydrargillite, etc. un-
doubtedly exists under certain circumstances. Max Bauer 3 has
apparently shown the presence of hydrargillite in the laterite
of the Seychellian Islands, and van Bemmelen 4 evidently regards
the mineral as a normal final product of the weathering of alumi-
nous rocks. Liebich 5 on the other hand, states, as a result of his
studies on beauxite, that alumina is not liberated from silicates

1 See Anleitung zur Mineralogischen Bodenanalyse, etc., by Franz Stein-
riede, Inaug. Dis. Friedrichs-TJniversitat Halle-Wittenberg. Halle, 1889.

2 See papers by MM. Delage and Legatu, in Comptes Kendu, Vol. 139,
1904, p. 1043, and Vol. 140, 1905, p. 1555.

"Neues Jahrb. fur Min. u. Petrog., 1898, Vol. II, p. 163.

4 Zeit. Anorgan. Chemie, Vol. 42, 1904, p. 265.

B Zeit. Prakt. Geol., 1897 (as quoted by Cameron and Bell).

MINERAL NATURE OF SOJLS 363

by ordinary weathering agencies. Although no special investi-
gations along these lines have been carried on by the United
States Bureau of Soils, it is nevertheless stated 1 that alumina or
aluminum hydrate is but seldom, if ever, a normal constituent
of soils. It is evident, therefore, that further investigations are
necessary before the matter can be regarded as definitely decided.
Since the work of Lemberg was made public, 2 it has been very
commonly assumed that various minerals of the zeolitic group
were present and exercised an important function in the con-
servation of soil fertility. Notwithstanding the somewhat en-
thusiastic endorsement by Hilgard, of this idea, as set fortja
in the previous pages, the writer can but feel that too much has
been assumed, both regarding their actual presence and their
possible utility.

One must not lose sight of the fact that the actual occurrence
of zeolites in soils is as yet not proven. Their presence is inferred
from the fact that weak acids, such as are known to be capable
of decomposing zeolitic minerals, will extract from the soil cer-
tain constituents which are characteristic of minerals of the
zeolitic group; and it is assumed, purely for lack of a better rea-
son, that these elements are those thus combined. Even if this
be true, their efficacy as potash holders may well be questioned,
since potash is not as a rule an element of great importance in
zeolitic minerals. Out of the 23 known species of zeolites (in-
cluding apophyllite), in but five is potash considered an essential
constituent. These five, as already noted on p. 29, are apo-
phyllite, ptilolite, mordenite, phillipsite, and harmotome, of
which phillipsite alone carries upwards of 6% (theoretically),
the other smaller amounts, the average for the five being about
4%. Now assuming that all the zeolites in the soils belonged
to these five groups and none to the 18 potash-free varieties,
and that 10% of any soil consisted of zeolitic material, even then
we have thus combined only 0.4% of K 2 O.

It must be remembered, further, that the zeolites are invariably
secondary minerals, as already noted, and as such are com-
monly regarded as decomposition products. This does not
necessarily mean, however, that they are products of superficial

1 Bull. 30.

2 Zur Kenntniss der Bilclung und Umbildung von Silicaten, Zeitschrift der
Deutschen Geolischen Gesellschaft, Vols. XXXVII and XXXVIII, 1885 and
1887.

364 THE REGOLITH

weathering. Indeed, in the majority of cases the evidence is
all to the contrary; they are plainly a result of deep-seated
processes going on in the rock masses long before atmospheric
action began to manifest itself. (See under Hydrometamor-
phism, p. 152.) Indeed the conditions prevalent in soil are un-
favorable rather than otherwise to the formation of zeolitic com-
pounds, and it is more than probable that such traces as there
exist are residuals from the breaking down of rock masses in
which they had been previously formed.

It is well to recall here the work of Curt von Eckenbrecher, 1
who showed by a series of analyses, in part his own and in part
those of Struve, Gmelin, and G. vom Rath, that in the early
stages of the weathering of phonolites there does seemingly result
a zeolitic product. The "weathering" in all these cases had, how-
ever, gone no farther than the formation of a whitish but still
firm and intact crust or zone about the unaltered material.
While the presence of the zeolite (natrolite) was not proven ab-
solutely, its formation from so readily altered a mineral as
nepheline or sodalite during the preliminary stages of weather-
ing, in which hydration is the most important factor, seems emi-
nently probable. It should be noted, however, that F. E.
Wright, 2 in working on what seemed a similar alteration product
in tinguaites from Cape Frio, Brazil, was unable by analysis to
show it to be other than a hydrated feldspar.

In this connection it is well to remember that zeolites as a
whole are characteristic of basic eruptive rocks, such as have
yielded but a proportionately small amount of our soils. Also
that the mutual chemical reactions that may go on in a rock
mass due to close juxtaposition of the various minerals may
largely cease in a soil where the amount of interspace is so enor-
mously exaggerated.

The researches made during the Challenger Expedition 3 showed,
it is true, that even at so low temperatures as from 2 to 3 C.
phillipsite is being formed in the deep-sea muds of the Central
Pacific and Indian oceans. But in these cases the mud is the
finely comminuted debris from basic eruptive rocks, itself pe-
culiarly liable to decay, and containing all the materials neces-

1 Tschermak 's Min. u. Pet. Mittheilungen, Vol. Ill, 1881.

2 Tschermak 's Min. u. Pet. Mittheilungen, Vol. 20, 1901, p. 29.

8 Rep. on the Scientific Results, 1873-76, Deep-sea Deposits, 1891, pp.
400-411.

SOLUBLE CONSTITUENTS OF SOILS 365

sary for zeolitic formation. It is, moreover, in a condition of
continual moisture, shut off from the oxidizing influence of at-
mospheric air, and under the weight of the thousands of fathoms
of overlying water which is here in a state of extreme quiescence,
being beyond the influence of superficial movements, as waves,
tides and currents. These conditions are so widely different from
those which exist in the superficial parts of land areas, that
they can be regarded as merely suggestive. The same may be
said relative to the zeolite (phillipsite and apophyllite) for-
mations at Plombieres as described by Daubree. 1 Another fact
which militates against the theory of zeolitic formation in soils,
is the almost universal absence of these minerals in such secon-
dary, unmetamorphosed rocks as are the product of the recon-
solidation of the same class of materials as in their unconsolidated
condition form soils. If they once existed, it would seem strange
they have not in some cases at least survived. If formed in
soils, why should they not be formed in secondary rocks where
the conditions are apparently so much more favorable?

It would, to the writer at least, seem more probable that the
soluble potash of soils exists, not in zeolitic combination, but
in some of the numerous decomposition products of feldspar,
nepheline, scapolite, etc., to which the name pinite is commonly
applied. Such at least is the case in the potash-rich soils of
Maryland, examined by R. L. Packard. 2 It is possible also that
it may exist in compounds allied to glauconite. More probable
yet is the supposition that their absorption and retention is due
to the colloidal condition into which the silicate minerals have
been shown 3 to pass under the influence of water and other agents
of decomposition.

The writer has elsewhere 4 pointed out that, particularly
among basic rocks, there may be actually a larger percentage
of matter soluble in hydrochloric acid and sodium carbonate
solution in rocks ordinarily designated as fresh, than in the
debris resulting from their decomposition. This fact he has
since emphasized in a paper read at the December (1896) meet-
ing of the Geological Society of America, and from which the
following statements are drawn. Rock-weathering, it must be

1 Geologic Experimental, pp. 180 et seq.

2 Bull. 21, Maryland Agricultural Experiment Station, 1893.

8 See Bull. 92, Bureau of Chemistry, and 30, Bureau of Soils. IT. S. Dept.
of Agriculture.

'Bull. Geol. Soc. of America, Vol. VII, 1895, p. 355.

366

THE EBGOLITH

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PHYSICAL CONDITION OF SOILS 367

remembered, is in the majority of instances accompanied by a
leaching process, whereby original soluble compounds, or new
soluble compounds formed during the process of decomposition,
are gradually removed. The final result is therefore, as already
many times noted, a residue consisting of the least soluble con-
stituents, and which forms the ordinary surface soil. Even in
cases where the actual amount of soluble matter is greatest in
a soil, the apparent excess may be due to water of hydration
and to the large amount of sesquioxide of iron, the latter being
practically insoluble in meteoric waters so long as there is a
free supply of oxygen, though readily soluble in hydrochloric
acid. These conclusions are based upon the table on p. 366 in
which the total percentage loss on ignition, minus the ignition
in the insoluble residue, is tabulated with the soluble matter.

(3) Physical Condition of the Soil. Chemically, as previ-
ously noted, a soil differs from the parent rock in the amount
of leaching it has undergone, and in the finely comminuted and
more or less decomposed condition of its particles. There are
other distinctions, not the least important of which are its state
of loose coherency and porous condition due to interstitial air
spaces. It has been estimated by Whitney 1 that the approxi-
mate number of grains in one gramme of soil varies between
2,000,000 and 15,000,000, the lowest estimate being that for a
sandy soil containing only some 4.77% of material in such an
extremely fine state of comminution as properly to be classed
as clay, while the highest number is that in a sub-soil contain-
ing some 32.45%. Our interest in these remarkable figures is
still further heightened when we are called upon
that these grains are not in actual contact, out
from the other by thin films of moisture, or, iiTory soil, by
actual air spaces. That such spaces exist is easily proven by the
fact that any soil may be greatly diminished in bulk by pressure.
The amount of this empty space is naturally quite variable, but
it is estimated to constitute on an average some 50%, by volume,
of the soil. That is to say, a cubic foot of soil, in its natural
condition, contains an amount of space between its grains, filled
by air or water, equal to one-half the entire mass.

These figures are given, not merely to illustrate the won-
derful degree of comminution reached in rock-weathering, but,
also, and what is of more importance from the standpoint of

1 Bull. No. 4, U. S. Dept. of Agriculture, Weather Bureau.

368 THE EEGOLITH

an agriculturist, the amount of surface exposed to the solvent
action of roots and percolating waters. Indeed, it has been
estimated that the total surface areas of the grains in a cubic
foot of soil amounts, on the average, to 50,000 square feet.
The amount is of course greater in a fine than a coarse soil, but
in any case sufficiently large to enable us to understand how,
under the ordinary conditions of cultivation, all the materials
essential to plant growth may in a brief time be removed, unless
renewed by artificial fertilizers.

Further than this, the amount of space between the grains
is of very great importance in determining the circulation of
water in the soil, and its capacity for retaining the right propor-
tion essential to plant growth as noted later.

The experimental work of late years goes to show that fertil-
ity is dependent upon these physical properties perhaps even
more than upon chemical composition. If the structure, i. e.,
the manner of arrangement of the soil particles, is such as to be
most favorable to root action and conservation of moisture, there
are few soils but may be made fertile by proper treatment, even
cannot the desired physical properties be imparted by artificial
means. A soil which contains too large a proportion of fine
clay matter may hold so large a proportion of moisture as to
be quite unsuited for cultivation when saturated, and become
equally unfitted by induration when dry. A light, porous,
sandy soil on the other hand, though fertile during seasons of
abundant precipitation, parts with its moisture so readily as to
be quite barren in seasons of drought. Porosity and capillarity,
two properties dependent wholly on the size ajid shape of the
soil particles, are therefore very essential items in this consid-
eration. Moisture precipitated in the form of rain soaks into
the ground or flows off upon the surface in varying proportions,
according to local conditions, an open porous soil naturally
absorbing more rapidly than one that is close and compact. 1

When, after the rain ceases, evaporation sets in from the
surface, the water which has soaked into the ground is brought
back again in part, by capillarity, though a part escapes through
leaching downward beyond the reach of capillarity, ultimately

1 The relative ' ' run off " of water to rainfall in the humid east has been
calculated as from 25% to 40% ; in the Cordilleran region 20% to
25% and in the arid region as from to 20%. Of the total rainfall from
50% to 100% is controlled by the belt of weathering. (Van Hise.)

PHYSICAL CONDITION OF SOILS 3G9

coming to the surface, at lower levels, in the form of springs.
The capacity of a soil to care for the water it receives from
rains is, perhaps, the most important of any one property.
It has been demonstrated that the soils of the semi-arid regions
of the West will produce abundant crops of wheat and corn,
though receiving but about half the amount of water from rain-
fall that would be requisite in the East. This is accounted
for wholly on physical grounds, and is explained as follows: 1
Water falling upon a perfectly dry soil descends very slowly,
and indeed, in extreme cases, may continue to fall for hours
without wetting the mass for more than a few inches below the
surface, while it will be absorbed very rapidly by a soil already
wet but not saturated. This is due to the fact, as explained
by Whitney, that in a dry soil the tension or contracting power
of the surface of the water is greater than the attraction of the
soil grains. If, on the other hand, there is any appreciable
amount of moisture in the soil, the tension of the water sur-
face will cause it to contract and pull the water from above
into the sub-soil. It follows, then, that the water of rains fall-
ing in semi-arid regions will not penetrate into the dry sub-
soil, until the overlying portions have become successively so
far saturated that they can no longer hold the water back,
and it will pass downward, therefore, very gradually into the
lower depths, saturating, or nearly saturating, each successive
depth as it progresses. Unless, then, as rarely happens in this
region, the rainfall is so great and so continuous as to saturate
the soil to a considerable depth, the whole supply of moisture
absorbed will remain within a short distance of the surface,
either immediately within reach of plant roots, or where it can
be brought upwards once more by capillarity when evaporation
from the surface begins. With a continuously wet sub-soil,
however, as in the East, a very considerable portion of the
water passes at once to depths beyond the reach of roots or
capillary attraction, and is, so far as our present considerations
are concerned, completely lost until, in the course of nature's
endless cycle, it shall once more be returned as rain. Within
certain limits, a light rainfall, equitably distributed, is more
advantageous to agriculture than are the heavier precipita-

Conditions in Soils of the Arid Region, by Milton Whitney, Yearbook
U. S. Dept. of Agriculture, 1894.
25

370

THE EEGOLITH

tions which characterize the Atlantic slopes of the American
continent.

The capacity of soils for moisture has been the subject of
experiment, and is found to vary widely, being naturally largely
dependent upon the size of the individual particles and the con-
sequent amount of interspace. Whitney states 1 that sub-soils
of Maryland truckland having 45% of interspace will hold but
22.41% by weight of water, when this space is completely filled.
The sub-soil of the Helderberg limestone, having 65% of space,
will hold 41.22%. King 2 gives the following table to show
the actual amount of water held by field soils when their sur-
faces are only 11 inches above standing water, this water having
been lifted into them by capillarity:

AMOUNT OF WATER IN SOILS

SOIL

PER CENT
OF WATEE

POUNDS OF
WATER

INCHES OF
WATER

Surface foot of clay loam contained . . .

32.2

23.9

4.59

Second foot of reddish clay contained . .

23.8

22.2

4.26

Third foot of reddish clay contained . .

24.5

22.7

4.37

Fourth foot of clay and sand contained

22.6

22.1

4.25

Fifth foot of fine sand contained ....

17.5

19.6

3.77

Total

110 5

21 24

According to Meister, different soils show water-holding capaci-
ties as follows: 3

WATER HOLDING CAPACITY OF SOILS

KIND OF SOIL

PER CENT
OF WATEK

IMBIBED

KIND OF SOIL

PER CENT
OF WATER

IMBIBED

Clay

50.0
60.1
70.3
63.7
69.0
59.9 *

Chalk

49.5
52.4
45.4
65.2
46.4

Loam
Humus

Gyseous .

Sandy (82 % sand) . . .
Sandy (64 % sand) . . .
Pure quartz sand ....

Peat .

Garden

Lime . . .

1 Some Physical Properties of Soils, Bull. No. 4, U. S. Dept. of Agricul-
ture, Weather Bureau, 1892.

2 The Soil, p. 159.

3 Handbook of Experiment Station Work. U. S. Dept. of Agriculture,
1893, p. 317.

KINDS OF SOILS 371

(4) The Weight of Soils. This is dependent upon (1) the
character of the particles composing the soil and (2) their
degrees of compactness. The figures given below are those of
Schubler. 1

WEIGHT PER CUBIC FOOT IN POUNDS, OF VARIOUS SOILS

Dry siliceous or calcareous sand 110

Half sand and half clay 96

Common arable soil 80-90

Heavy clay 75

Garden mould, rich in vegetable matter 70

Peat soil 30-60

(5) Kinds and Classification of Soils. Being derived from
rocks of all kinds and under greatly .varying conditions; in
almost infinitely variable conditions of comminution, decay, and
proportional amounts of their various constituents, no hard and
fast lines for soil classification can be laid down. All things
considered, they are best classed with the regolith of which they
form a part, the general divisions of which are given in tabular
form on p. 288. We thus have the primary divisions of seden-
tary and transported soils, accordingly as they have been formed
in place, or transported. Each of these is again subdivided ac-
cording to the agencies involved in its transportation or original
formation.

Many varietal names have been applied to soils, but as a rule
in so loose and ill-defined a manner as to give them only a very
general significance. A common practice is to name one of
sedentary origin according to the rock from which it was de-
rived, as granite soil, limestone soil, etc. Transported soils, on
the other hand, are often designated either by the agencies in-
volved in transportation, as glacial, or ceolian soils, their position,
as terrace soils, or their physical or chemical characteristics, as
sandy or clayey soils. A loam is usually defined as an admixture
of sand and clay with more or less organic matter, a clayey
loam being one in which clay predominates and a sandy loam
one in which sand prevails. The terms peat, muck, loess, marl,
etc., have been already sufficiently defined. Local names indica-
tive of suitability for particular crops, or sometimes of doubt-
ful or obscure meaning, are frequently met with. The bluegrass
soils of central Kentucky are limestone residuals celebrated for

1 Handbook of Experiment Station Work, U. S. Dept. of Agriculture,
1893, p. 315.

372

THE EEGOLITH

the luxuriant growths of Poa pratensis which they bear. The
red "buckshot" soils of the Yazoo bottoms, Louisiana, are stiif
clayey alluvial soils mottled with ferruginous spots.

Many names indicative of mode of formation have already
received attention, but a few others may be here noted. The
names Endogenous and Exogenous have been proposed for
soils formed in place (sedentary) or derived from other sources
(transported). It is presumably scarcely necessary to remark
that such terms are quite inapplicable and inappropriate.

The name regur is locally applied to a fine dark argillaceous
soil particularly suited for cotton growing which has a wide
areal distribution throughout southern India. Its origin ap-
pears to be mainly subaerial, though a part of the material so
called is undoubtedly alluvial. The material is highly plastic
when wet, and expands and contracts to a remarkable degree
under varying conditions of moisture and dryness. This soil
is very retentive of moisture and rarely requires to be irrigated
artificially. It is, as a rule, of great fertility and of wonderful
lasting powers, it being stated that in some localities it has
borne crops for 2000 consecutive years, without the aid of ma-
nures. In depth this soil is rarely over 6 to 8 feet. The follow-
ing analyses show the chemical character of the regur (from
near Seoni) on the surface and at depths of (An) 5 feet and
(Bn) 3 feet below. The analyses A are instructive as showing
the large increase in the amount of lime from the surface down-
ward. Although not so stated, the slight differences in Bi and
Bn are probably due to the lesser depth below the surface from
which Bn was taken.

CHEMICAL ANALYSES OP EEGUR, OF INDIA

{

Insoluble matter

62.7%

47.61 %

62. 8%

63.7%

Organic matter
Water

9.2
8 4

8.4

7 6

9.0

8.2

8.7
6.5

Oxide of iron

11.0

15.9

10.9

11.8

Alumina

7.5

8.6

7.6

8.4

Carbonate of lime .

1 2

11 89

1.5

1.3

100.00%

THE COLOK OF SOILS 373

In many cases this regur is derived directly from basaltic
rocks, by surface decomposition in situ, whilst other varieties
were derived from other aluminous rocks, or are alluvial deposits
in river valleys, lakes, lagoons, and marshes. The dark color, as
is usual, is due to the presence of organic matter. 1

The term sub-soil is applied to that portion of the regolith
which immediately underlies the soil proper, from which it
differs mainly in compactness, and the smaller amount of oxi-
dation and decomposition it has undergone. In a soil which
has never been cultivated, the sub-soil may pass gradually up-
ward into the soil without distinct lines of demarcation. Pro-
longed cultivation may, however, have so thoroughly oxidized
and physically altered the superficial portions down to the limit
of plough and root action, as to bring about a very marked differ-
ence, both in color and texture, as well as in actual composition.
At times the sub-soil becomes so thoroughly compacted as to be
almost impervious, forming a so-called hardpan.

(6) The Color of Soils. The color of soils is due mainly to
carbonaceous matter and iron oxides. To the first are due the
dark gray to black colors characteristic of prairie and swamp
soils. To iron oxides are due the buff, yellow, ochreous-brown,
and red hues, the source of the oxides being mainly the silicate
minerals from whence the soils were derived. It sometimes
happens, as abundantly demonstrated in the southern Appa-
lachian states, that it is possible in passing over any section
of the country to designate with a fair degree of accuracy
the lithological nature of the underlying rocks from the color
of the residual soils, even though the rocks themselves may
be wholly obscured by decomposition products. In such cases
rocks rich in iron silicates, like hornblende and augite, give
rise to bright red soils, while those poor in these constituents
yield soils of a gray or slightly yellowish hue. Much, however,
depends on extent of decomposition and on climatic conditions,
as noted below.

One of the most striking features of the landscape observed
in traveling southward along the Appalachian belt is the abrupt
transition in color of the soil, as the limit of glacial action is
past. Within the glaciated area, except where derived directly
from highly colored rocks, like the Triassic sandstones, the soils
are everywhere dull in color, some shade of gray, drab, or brown.

1 Manual of the Geology of India, 2d ed., by R. D. Oldham, 1893, p. 411.

374 THE EEGOLITH

South of this limit ochreous-red and yellowish prevail. Along
the line of the Virginia railways south of Washington, these col-
ors prevail in hues of surprising brilliancy. Although the soils
throughout the region are residual, their colors seem quite inde-
pendent of the kind of rock to which they owe their origin.
Granite, gneiss, schist, or basic trappean rocks alike give rise to
red and yellow highly tenacious residues of such depth and bril-
liancy of color that every gully, ravine, and roadway stands out
against the green background of the landscape, as though painted
by some Titanic hand with brushes dipped only in yellow, red,
and vermilion ochres. These contrasts are particularly striking
in the early summer and directly after a rain. But he who
wishes to admire had best do so from his window, and without
too much attention to detail.

The soil is plastic and adherent to an intolerable degree. The
grass forms no compact sod, as in the North, and as a result
the walls of houses, fences; feet, legs and clothes of pedestrians
become stained a dirty ochreous color equally trying to the
housewife and to ploughman.

The cause of this color variation has been the subject of
discussion by Professors Crosby, 1 Dana, 2 Russell, 3 and others.
So far as our knowledge now extends, it is apparent, as first
stated by Crosby, that the difference is due to a spontaneous
dehydration which takes place in the warmer regions, whereby
the hydrous sesquioxides of the type of limonite and gothite
are converted into the less hydrated or anhydrous forms tur-
gite and hematite with corresponding changes in color from
yellow or brown to red.

This view is rendered the more plausible from the fact that
the most brilliant hues are quite superficial, and below the sur-
face, fade out gradually into brown and yellow or even gray hues.
Such a transition may be observed in any fresh road cut, but
quickly become obscured by. the washing down of the deeply
colored material from the higher levels. Sometimes the brilliant
red will be found a mere wash, but a fraction of an inch in
thickness, or again it penetrates to the depth of a foot or more
before giving way to more modest hues. In such cases the

1 Proc. Boston Society of Natural History, 1885, p. 219, and Technological
Quarterly, Vol. IV, 1891, p. 36. *

2 Am. Jour, of Science, Vol. XXXIX, 1890, pp. 317-319.
8 Bull. No. 52, U. S. Geol. Survey, 1889.

THE AGE OF SOILS 375

brilliant colors will be found to have penetrated deepest along
joint lines, or the more porous portions, leaving the intervening
compact masses of more sombre hue. -

In discussing this matter, there is, however, one point that we
should not overlook, although its importance seems not to have
been fully realized by the authorities quoted, and that is, a
change in color due not alone to a change in the conditions of
the iron, but to the relatively greater abundance of this constitu-
ent in the uppei portions. The iron oxides, as already noted,
owing to their less soluble nature accumulate in the residues,
and as a rule, the more thorough the decomposition the greater
the proportional amount of iron. A small percentage of free
oxide disseminated throughout a relatively large amount of
detritus imparts but little color; the more iron, the more color.
The residue from the Medford diabase described on p. 200 is
of a deep brown color, as a whole, but the finest silt washed
from it is several shades brighter, of a dull ochreous red. Had
the entire mass decomposed to the condition of this silt, we
might expect it to have the same color. This change, due to
increased proportional amounts of iron oxides, is particularly
marked in limestone residuals where the original rock may con-
tain merely traces of free oxides, or ferruginous silicates. Neu-
mayer has shown 1 that the snow-white Karst limestones contain
only some 0.044% of ferruginous silicates which themselves carry
20% of iron oxides. Yet the residual soil left by the decompo-
sition of this limestone is of so pronounced a color as to have
long ago received the name terra rossa or red earth.

Other things being equal, brilliancy of color may then be
regarded as (1) indicative of advanced decomposition, and (2)
of geological antiquity.

(7) The Age of Soils. No sooner were the first rocks pushed
above sea-level than the various agencies described under the
head of weathering began the work of disintegration, decompo-
sition, and transportation. Of this we have ample proof in the
entire series of sedimentary rocks extending from the Archaean
down to the most recent and which are but the reconsolidated
residues of pre-existing masses. That such a breaking down
resulted in the production of soils is a fair inference, though
we have no absplute evidence of land plants and hence, a
priori, of soils, before the beginning of the Upper Silurian

1 Verhandl. k. k. Geol. Reichsanstalt, 1875-76, p. 55.

376

THE EEGOLITH

period, when plants of the lycopod type appeared. Such soils,
as soils, have, however, long since disappeared in the never-
ending cycle of change, and it is not until we reach the Car-
boniferous period that we meet with soils which have been
preserved in place and in recognizable form even to the present
day. Even here induration and partial metamorphism has
rendered them no longer fitted for the support of plant life,
but that they once did so serve is amply proven by the occa-
sional finding of erect, fossil tree trunks with roots buried in
their native soil, as they grew in the marshes and woodlands of
the coal period. But as to the time of the beginnings of the
formation of such soils as still retain their soil characteristics,
we have not in all cases reliable data. Those which are but the
unconsolidated sediments of recent geological time, like those

of the eastern shore of Mary-
land, the loess and alluvium
of the Mississippi valley, or
the swamp and glacial soils of
the north and east may, of
course, be located with a rea-
sonable amount of accuracy.
But as for the residual soils,
those which result from the
breaking down in place of
rock masses, we can only say

FIG. 38. Trunk of tree still standing
in soil of Carboniferous age. a,
bed-rock; 6, under clay or ancient
soil; c, coal; d, bedded rock; e f
fossil tree.

that they must be younger than
the rocks from which they
were derived. The writer has shown that the granite soils of the
District of Columbia are post- Cretaceous ; in other parts of the
Piedmont plateau of Maryland, they may be post-Tertiary. In
but few instances, as at Medford in Massachusetts, have we evi-
dence of any considerable amount of soil formation by decom-
position and disintegration since the close of the glacial period.
Obviously the older a residual soil, the greater the amount of de-
composition and leaching it will have undergone and the less
will it resemble the parent rock. Where horizontally lying
strata of varying character have successively undergone decom-
position and a loss of their soluble constituents, the resultant
soil must periodically vary according to the nature of the rock
undergoing decomposition and the inherited characteristics

THE AGE OF SOILS 377

handed down from the strata earlier decomposed. In such a case
as that figured on p. 291, we have a residual soil containing the
least soluble constituents of the hundreds of feet of dissolved
and disintegrated rock which once extended across the entire
country, becoming commingled with that now undergoing, in

. PoarSoil

FIG. 39.

its turn, the soil-making process. Such a soil may, therefore,
in extreme cases, contain materials of all ages from the first
product of disintegration of the uppermost strata, which may
have been Carboniferous, to that which formed to-day, and may
be Cambrian.

It is, of course, true that through the erosive action of water
these soils are continually losing their finer silt and clay-like
particles, it may be almost as fast as formed, especially in hilly
regions, and that as the soil drops lower and lower in the geo-
logical horizons indicated, it becomes more and more impover-
ished in those constituents derived from the upper beds. But
as to what proportion of the material of one horizon is handed
down to become admixed with that from the rocks below, we
have no means of judging, and in fact it must be ever-varying.

The matter of the geological age of any soil, or the age of
the rocks from which it was derived, is therefore of only very
general interest, and may well be dismissed here. The attempt
which has been made by another writer 1 to discriminate or
classify soils according to the geological horizons of the rocks
from which they were derived, is believed by the present writer
to be futile and wholly inexpedient.

No attempt should be made, as has been done by at least one
writer, to state the character of soil that may arise from the
weathering of any particular class of rocks, since much depends

*See Stockbriclge 's Rocks and Soils, p. 12.

378 THE EEGOLITH

upon the extent to which weathering has been carried. TL'.
ultimate product of weathering of rocks of any but the purel^
siliceous type is a more or less ferruginous clay, which maj
be contaminated or admixed with coarser, foreign particles. It
is the extent of decomposition, more than its lithological deriva-
tion, that determines both the chemical composition and physical
characteristics of any soil.

Rocks of essentially the same type so far as composition is
concerned, regardless of structural modifications induced by
either to the Archaean or older Palaeozoic formations, but this
metamorphism, have been formed and re-formed throughout
every period of the earth's history. As has been already
indicated, the greater portion of the granitic, gneissic, or highly
metamorphosed crystalline schists and calcareous rocks belong
merely because they, being older, have been longer subjected to
metamorphosing agencies, and not because in themselves they
possess essential differences. It is true that some authorities
lay stress on the supposed abundance of animal remains in cer-
tain Palaeozoic formations, but no one but the veriest amateur
would now dream of attempting to discriminate between either
igneous or aqueous rocks of the same nature, but of different
geological ages, on purely chemical grounds.

It is a fact, however, that within certain climatic limits, the
rocks of any one horizon may impart such characteristics to a
residual soil as shall render it adapted to plant growth of a
particular kind. Thus, 1 throughout the central portion of Ken-
tucky, vriiere, within the distance of a few miles, rocks occur of
several distinct geological horizons, each bearing its mantle of
residual soil, each horizon may be traced for long distances,
though the rocks themselves are wholly obscured, merely by the
character of its forest growth. This feature is, however, prob-
ably dependent more on physical than chemical characteristics.

(8) Soils as affected by Plant and Animal Life. There are
various forms of animal and plant life the action of which is
worthy of note in connection with the subject of decomposition ;
but since it is probable that they are of greater efficiency in
promoting changes in soils once formed than in bringing about
the preliminary rock disintegration, their consideration has been
left to form a portion of the present chapter.

Ants, by means of their numerous borings, penetrating at

1 As the writer is informed by Mr. J. E. Proctor.

PLATE 31

Gullied field, near Marion, North Carolina.

EFFECT OF PLANT AND ANIMAL LIFE 379

times to depths of many feet, bring about not merely a rear-
rangement of soil particles through a transfer of materials from
lower to higher levels, but also a condition of porosity whereby
air and water gain access to the deeper lying portions, there
to promote further chemical and physical changes.

Naturally these insects limit their work to dry and light
soils, where their operations may be compared with that of earth-
worms whose operations are confined to moist ones. Shaler has
calculated 1 that over a certain field in Cambridge (Massachu-
setts) the ants have made an average transfer of soil matter
from the depths to the surface sufficient to form a layer each year
of at least a fifth of an inch over the entire four acres under
observation. He further
mentions a curious effect
arising from the interfer-
ence of the ants with the
original conditions, in the
separation of the finer from FIG. 40. Effects of ant-hills on soils, aa,
the coarser particles. In sand accumulated in hill; &&, material
certain parts of New Eng- washed down the slopes, mingled with

vegetable mould,

land where sandy soils had

laid for a long time uncultivated, fields were covered to a depth of
some inches with a layer of fine sand without pebbles larger than
the head of a pin, while below the level of perhaps a foot the
soil was mainly pebbles, with very little finer material. This
condition, it is argued, was brought about by the tens of thou-
sands of ants which each year, over every acre, in the process
of building their dwelling brought up the finer material and
deposited it in the form of a mound about the surface openings,
leaving behind the coarser particles, too heavy for them to
move. The common black and brown ants of the United States
(Formica exsectoides) build upon the surface mounds in many
cases from 1 to 2 feet in height, and 3 to 5 feet in diameter,
which are composed of materials brought up from below the
surface intermingled with twigs and shreds of bark and leaves.
The mass of some of these mounds has been calculated as equal
to 2 cubic yards. Being of unconsolidated, loosely coherent ma-
terial, such are constantly being degraded by wind and rain and
their particles distributed over the surrounding surface. ' * Where

1 12th Ann. Eep. U. S. Geol. Survey, 1890-91, p. 278.

380 THE EEGOLITH

these structures are numerous, as they are in certain districts
in the United States, by their constant deposits of matter on
the surface of the ground, they bury a good deal of vegeta-
ble waste in the soil, at the same time the animals are con-
stantly conveying into the earth large quantities of organic
matter which serves them as food, and the waste of this,
including the excreta of the animals themselves, is of con-
siderable importance in the refreshment of the soil." The geo-
logical efficacy of insects of this and other types is undoubtedly
greater in warmer climes, where not only are they found in
greater abundance, but their period of activity extends over a
larger portion of the year. Messrs. Mills and Branner, as al-
ready noted, are inclined to lay considerable stress on the work
of ants and termites in bringing about soil changes and rock
decomposition in Brazil. Branner states that in some parts of
.the Amazon valley, of Minas Goyaz and Matto Grozzo, the soil
"looks as if it had been literally turned inside out by the bur-
rowing of ants and termites." The species popularly known
as saubas excavate chambers and build galleries which are fre-
quently from 50 to 100 feet long, from 10 to 20 feet across, from
1 to 4 feet high, and contain tons of earth. The white ants or
termites, like the true ants, burrow extensive channels in the
ground, and build up huge nests upon the surface from the
size of which one may gain some idea of the extent of the under-
ground galleries. In the region extending from the state of
Parana to north of the Amazon and along the upper Paraguay
in Matto Grosso may be seen places where the nests are so close
together that one can almost walk upon them for several hun-
dred yards at a time, while no one of the nests is more than 10
feet from another over many acres of ground. Such vary in
size from 1 to 12 feet in height and 1 to 10 feet in diameter, and
do not seem to be confined to any particular kind of country,
though especially noticeable in the interior and timberless re-
gions. The constant transference of such quantities of soil from
below to the surface, and of organic matter from the surface
downward, cannot fail to bring about marked changes in its
physfcal as well as chemical condition, while at the same time
affording passageways for air and meteoric waters, as already
noted. 1

a ln a later paper (Journal of Geology, Vol. 8, 1900) Prof. Branner de-
scribes the ant-hills, about Urucu station as "so thick that the county looks

EFFECT OF PLANT AND ANIMAL LIFE 381

Certain animals, like the crayfish, have likewise a habit of
burrowing in the ground, though as they are wholly subter-
ranean or aquatic in their nature, the results are less conspicu-
ous to the casual observer. In searching for their food, these
animals bore numerous horizontal channels or galleries some-
times an inch or so in diameter and extending for many feet,
usually ending in an upward shaft reaching to the surface, or
the margin of a pond or stream. These form natural drainage
channels and allow a more ready access of air, converting what
might under other conditions be a heavy, clayey or even marshy
soil, unfit for cultivation, into one light and fertile.

By burrowing through dams and embankments, they have,
however, in some instances so weakened these structures as to
cause them to give way, and large districts have become inun-
dated and rendered unfit for cultivation.

Probably none of the forms of animal life thus far mentioned
produce such wide-spread and beneficial results as have been
ascribed by Darwin 1 to the common earthworm, the angleworm
of the New England disciples of Isaac Walton. These insig-
nificant creatures burrow in the moist rich soil, and derive
their nourishment from the organic matter it may contain. In
order, however, to obtain this comparatively small amount of
nutritive matter, they devour the earth without any selective
power, and pass it through their alimentary canals, rejecting
the non-nutritious portions, which nearly equals in bulk that
first taken in. The numerous holes made, while in part perhaps
to afford passage to the surface, are mainly excavated in this
process of soil eating and actually represent the amount of ma-
terial which the worms have passed through their digestive
systems.

Darwin states that in certain parts of England these worms
bring to the surface every year, in the form of excreta, more than
10 tons per acre of fine dry mould, "so that the whole superficial
bed of vegetable mould passes through their bodies in the course
of every few years." By collecting and weighing the excretions
deposited on a small area during a given time, he found that the
rate of accumulation was an inch in every five years. The

like a field of gigantic potato hills" and he expressed the belief that in
Brazil, and the tropics generally, ants are of more geological importance
than are earthworms in temperate regions.
1 The Formation of Vegetable Mould.

382 THE EEGOLITH

importance of the worms, both as mellowers of the soil and as
levellers of inequalities is therefore very great, and cannot be
overlooked here.

While the main influence of the worm is manifested in a
mellowing by burrowing and a transfer of material from a
lower to a higher level, they bring about a slight admixture
of organic matter through a habit of coming to the surface at
night time, and dragging down into their burrows small shreds
of leaves and grass, which, taken into account in connection with
the excrementitious matter of the worms themselves, must tend,
though it may be ever so slightly, to enrich the soil. The sub-
ject should not be dropped without referring to the abundance
of these worms, which in England has been estimated as at the
rate of 53,767 to each acre of garden land, and about one-half
that number for pasture land. It is scarcely necessary to re-
mark that their distribution is very unequal throughout the
world, and that in dry sandy regions they are almost, if not
wholly, unknown.

In northern temperate climates, such as that of New England,
and particularly where the soil is of a clayey nature like the
ground moraine, the burying action of the earthworm, as de-
scribed above, may be wholly overcome through the heaving
action of frost. Every farmer boy who has been condemned
to pick the drift boulders from a field knows through bitter
experience that, however well he may do his work in the fall,
however clean the surface may be when winter sets in, the fol-
lowing spring, after the frost is out of the ground, will find a
new crop in no way distinguishable from the old, which, for
all that he can see, may have rained down during the win-
ter's storms. The fact is, however, that they have been actually
thrown up, "heaved out," the farmers will say, from below the
surface by the frost which here penetrates to a depth of two or
more feet. As the water-soaked clay underlying one of these
buried boulders freezes, it expands upwards, since this is the
direction of least resistance. The stone is carried up bodily for a
distance dependent on the amount of expansion. When the
frost leaves the ground, the soil sinks back nearly to its first
position ; but the boulder never quite regains its former place,
being prevented by particles of soil, or clay or pebbles which fall
into the cavity as the soil shrinks away from it. The amount of
actual lifting for each season may be but slight, but as the

EFFECT OF PLANT AND ANIMAL LIFE 383

process goes on unceasingly there is always an abundance of new
material at the surface each succeeding spring. This heaving
action of the frost is abundantly exemplified in these clay regions
by the throwing out of fence posts and roots of leguminous plants
like clover. In wet boggy lands the heaving action of frost, as
exerted on partially buried boulders of small size, is sometimes
exemplified in a peculiarly striking manner. The surface of
the ground will be dotted here and there with small hummocks,
each with a comparatively large crater-like opening at the top.
Investigation reveals the fact that at a distance of but a few
inches at most below the surface of this crater-like opening is
a rounded boulder. The heaving action of the frost forces the
boulder gradually upward, causing the turf to first rise with
smooth rounded outline, till, through continual pressure from
the boulder, it bursts at the top. When the frost leaves the
ground, the boulder drops back a short distance, but enough
to be quite out of sight, leaving the cavity at the top filled
with mud, and looking in outline like a small mud volcano.
So far as the writers observations go, the heaving action rarely
progresses, in these areas, to the point of actually throwing
the boulder out upon the surface. Each summer the growing
turf makes an attempt at healing the wound, but each winter's
frost opens it once more, the alternating forces so nearly bal-
ancing that little is accomplished after this pseudo-volcanic
stage is reached.

Insects like the boring bee, the burying beetle, or larger bur-
rowing animals, like the "woodchuck" of the Eastern states,
the prairie dogs, badgers, and spermophiles of the West, exert
powerful though local influences in admixing the lower with the
upper portions of the soil, and through allowing perhaps a more
ready passage of water facilitating oxidation and decomposition
at greater depths. (Fig. 2, PI. 20.)

While the effect of these animals may be comparatively in-
conspicuous in the regions east of the Mississippi, in portions of
the drier regions of the West the surface is so undermined
by burrows as to make traveling on horseback at more than a
very moderate pace a matter of grave difficulty. W. P. Blake,
in the early reports of the Pacific Railroad Survey, stated that
the fine, silty soil of the Tulare valley in California is so under-
mined that it is almost impossible to travel over it. ' ' Mules often

384

THE KEGOLITH

break through the thin crust and sink to their shoulders in these
holes."

J. B. Hatcher states 1 that along the valleys and bluffs of the
smaller rivers of Patagonia rodents are extremely abundant and
often the entire earth for a depth of nearly two feet is literally
undermined over areas of many square miles with subterranean
passages which greatly impede the traveler, whose horse drops
in at every step, half way to the knee.

The action of plant life in the accumulation of vegetable
mould has been fully discussed under the head of cumulose

and alluvial deposits.

'// There is, however, one

phase of action which may
well be mentioned here. A
growing tree sends its roots
deep down into the earth in
Forest Mould, search of food and foot-
hold. So long as the tree

True Soil.

remains alive and standing,
sou. in firm soil the amount of
change in the soil itself,

Rock. , ,-, <? i

except in the way of ab-
straction of certain constit-
uents taken up by the
growing plant, is presum-
ably very small. When,
however, the tree dies,
[forest Mould, the roots slowly decay,
sou, an( j besides yielding up
Soil - their contents to form
new soil, afford passage-

riG> 42> ways for percolating

water with all its atten-
dant results. Moreover, cases are by no means rare in which
trees are upturned by the winds, bringing entangled in their
roots it may be tons of soil and boulders which in part gradually
fall back into the hole and in part remain to form a mound which
marks the spot long after the tree has decayed. Into the cavity
formed, dead leaves and other organic debris accumulate, which
in time form deep rich loam to be commingled with the stony

1 National Geographic Magazine, No. 11, 1897, p. 318.

FIG. 41.

EFFECT OF PLANT AND ANIMAL LIFE 385

matter of the soil. In sections of the country where heavy
winds and hurricanes are of frequent occurrence, the efficacy
of trees in thus burying organic matter, and producing a more
complete intermingling of the soils, is by no means inconsider-
able. 1 The influence of plants in adding carbon and incidentally
carbonic and other organic acids to the soils has been described
in previous pages. When plants die and decay upon the im-
mediate surface, there is left only the inorganic matter or ash
behind, the carbonic acid escaping into the air or being carried
by rains into the soil. Hence it would seem to naturally follow
that the soil where supporting an abundant vegetation should
contain a larger percentage of carbonic acid than the atmosphere
itself. That it does not contain, in all cases, a greater amount
of free carbonic acid is apparently brought out in the table from
the works of Boussingault and Lewy, quoted on p. 156.

Bacteria as agents of nitrification are undoubtedly efficacious
in preparing nitrogeneous matter in the soils for assimilation
by growing plants. Their influence as decomposers of rock
masses was noted on p. 181. According to Wiley, 2 it is highly
probable that organic nitrogen in the soil, in passing into the
form of nitric acid, exists at some period of the process in the
form of ammonia. The products of nitrification are ammonia,
nitrous or nitric acid, carbon dioxide, and water. The ammonia
and nitrous acid may not appear in the soils as the final products
of nitrification, as the organism attacks the nitrous acid at once,
converting it into the nitric form.

It may at first seem strange that man, who prides himself on
being the highest type in the animal kingdom, as well as the
only animal endowed with reasoning powers, should prove the
most destructive; yet such is the case. Through prodigality,
due in part to thoughtlessness and in part to a wilful disregard
for any but immediate interests, man has, apparently from the
very beginning of his existence, so conducted himself with re-
lation to natural resources as to leave little less than ruin in
his path. This is true not merely with reference to his treat-

1 Some of our archaeologists go so far as to assert that the stone imple-
ments found buried several feet below the surface in glacial deposits, and
brought forward as proving the existence of pre-glacial man, have been
brought into that position by just such agencies. See Holmes, Early Man
in Minnesota, American Geologist, April, 1893, p. 228.

'Principles and Practice of Agricultural Analysis, p. 464.
26

386 THE EEGOLITH

merit of the soil, but of the deeper lying rocks and their min-
eral contents. In the name of development he has squandered;
through careless husbandry he has not merely impoverished
the soil, but in many cases allowed it to run waste and be lost
beyond recovery. So long ago as 1846, when Lyell made his
second visit to America, he was struck by the rapid denuda-
tion of the land in our Southern states due to the reckless cut-
ting away of the forests. He described near Milledgeville, in
Georgia, a washout in a lately deforested area. "Twenty years
ago," he wrote, "before the land was cleared, it [the washout]
had no existence; but when the trees of the forest were cut
down, cracks 3 feet deep were caused by the sun's heat in the
clay; and during the rains, a sudden rush of water through the
principal crack deepened it at its lower extremity, from whence
the excavating power worked backwards, till in the course of
20 years, a chasm measuring no less than 55 feet in depth, 300
yards in length, and varying in width from 20 to 180 feet was
the result. The high road has been several times turned to
avoid this cavity, the enlargement of which is still proceeding,
and the old line of road may be seen to have held its course,
directly over what is now the widest part of the ravine. In
the perpendicular walls of this great chasm appear beds of clay
and sand, red, white, yellow, and green, produced by the de-
composition in situ of hornblendic gneiss, with layers of veins
of quartz, which remain entire, to prove that the whole mass
was once solid and crystalline. ' ' *

The same lack of foresight or wanton disregard for coming
generations is still manifested, and every muddy stream bears
downward to the sea an increased load of silt from lands im-
properly cultivated and from which every rain removes a por-
tion of the finest and richest of the soil, leaving behind but the
barren gravel, channelled it may be beyond the possibility of
cultivation. McGee 2 has more recently made observations of
a similar nature in southern Mississippi, where the softer loam
of the Columbia formation, which here forms the soil, has
been allowed to become eroded down to the barren sandy loam
of the Lafayette. "Old fields are denuded by the acre, leaving
mazes of pinnacles divided by a complex network of runnels
glaring red toward the sun and sky in strong contrast to the
rich verdure of the hillsides never deforested; the plantations,

1 Lyell, Principles of Geology, 9th ed., 1846, p. 204.
2 12th Ann. Eep. U. S. Geol. Survey, 1890-91.

EFFECT OF PLANT AND ANIMAL LIFE 387

mansions, and 'quarters' are undermined, and whole villages,
once the home of wealth and luxury, are being swept away at
the rate of acres for each year. ' '

"The ravages committed by man," writes Marsh, 1 " subvert
the relations and destroy the balance which nature had estab-
lished between her organized and her inorganic creations, and
she avenges herself upon the intruder by letting loose upon her
defaced provinces destructive energies hitherto kept in check
by organic forces destined to be his best auxiliaries, but which
he has unwisely dispersed and driven from the field of action.
When the forest is gone, the great reservoir of moisture stored
up in its vegetable mould is evaporated, and returns only in
deluges of rain to wash away the parched dust into which that
mould has been converted. The well-wooded and humid hills
are turned to ridges of dry rock, which encumbers the low
grounds and chokes the watercourses with its debris, and
except in countries favored with an equable distribution of rain
through the seasons, and a moderate and regular inclination of
surface the whole earth, unless rescued by human art from the
physical degradation to which it tends, becomes an assemblage
of bald mountains, of barren, turfless hills, and of swampy and
malarious plains. There are parts of Asia Minor, of northern
Africa, of Greece, and even of Alpine Europe, where the opera-
tion of causes set in action by man has brought the face of the
earth to a desolation almost as complete as that of the moon;
and though, within that brief space of time which we call 'the
historical period/ they are known to have been covered with
luxuriant woods, verdant pastures, and fertile meadows, they
are now too far deteriorated to be reclaimable by man, nor can
they become again fitted for human use, except through great
geological changes, or other mysterious influences or agencies
of which we have no present knowledge, and over which we
have no prospective control. The earth is fast becoming an
unfit home for its noblest inhabitant, and another era of equal
human crime and human improvidence, and of like duration
with that through which traces of that crime and that improvi-
dence extend, would reduce it to such a condition of impover-
ished productiveness, of shattered surface, of climatic excess,
as to threaten the depravation, barbarism, and perhaps even
extinction of the species. ' '

1 The Earth as modified by Human Action, a new edition of Man and
Nature, by. Geo. P. Marsh, pp. 43, 44.

LIST OF AUTHORS CITED OR RE-
FERRED TO.

Adie, Alex. J., 159

Agassiz, L., 153, 154

Alden, W. C., 341

Aughey, S., 181, 319

Bartlett, W. H., 159

Bayley, W. S., 70, 74, 81

Bauer, M., 362

Beaumont, Elie de, 138

Becker, G. F., 221, 289

Bell, J. M., 360

Bell, Kobert, 172, 229, 232, 262

Belt, T., 159, 248, 266, 272

Berthier, P., 223

Berthelot and Andre, 174

Bischof, G., 168, 19i, 223

Blake, W. P., 164, 233, 244, 334, 383

Blum, J. K., 75

Bolton, H. C., 181

Bonney, T. G., 232

Branner, J. C., 104, 153, 157, 166,

182, 264, 380
Broeck, Van den, 243
Brogger, W. C., work of, 60
Brongniart, A., 82, 153, 223
Buchanan, J. V., 183
Caldcleugh, A., 170
Cameron, F. K., 360
Cameron and Bell, 174
Chamberlin, T. C., 264, 292
Clarke, F. W., 4
Clark, W. B., 117
Choffat, P., 241
Collier, P., 356
Cox, E. T., 118

Crosby, W. O., 167, 241, 341, 374
Cross, Whitman, 32, 58, 67, 77
Culberson, 258
Cushing, H. P., 264
Dale, T. N., 149
Dana, J. D., 45, 53, 177, 221, 239,

249, 374

Darwin, Charles, 153, 219, 280, 381
Daubree, Gustav, 17, 174, 365
Davis, W. M., 164
Davidson, C., 275
Dawson, J. W., 279, 322
Delesse, A., 61

Derby, O. A., 167, 272

Dewey, F. P., 131

Diller, J. S., 83, 87

Dutton, C. E., j.75

Dwight, Timothy, 285

Ebelman, M., 205, 223

Ebermayer, 268

Eckenbrecher, C. von, 364

Ehrenberg, C. G., 124

Egleston, Thomas, 163

Ewing, A. L., 171

Failyer, G. H., 154

Fernow, B. E., 268

Fesca, Max, 229

Fischer, F., 157

Forchhammer, J. G., 191, 223

Fournet, J., 152, 221, 223

Fulton, E. L., 266

Furlonge, W. H., 272

Geikie, A., 2, 128, 179, 276

Geikie, James, 345

Geldmacher, Max, 223

Gesner, H. S., 305

Gilbert, G. K., 50, 164, 244, 336

Goodchild, J. G., 260

Gordon, C. H., 146

Griswold, L. S., 104

Gumbel, C. W., 84

Hall, C. W., 139

Harrison, J. B., 208

Hartt, F., 153, 266

Hatcher, J. B., 384

Hawes, G. W., 70, 83, 148

Hauy, E. J., 72, 76

Hayes, C. W., 172, 271

Heusser and Claraz, 153, 213, 237

Hilgard, 320, 333, 355, 357, 363

Hitchcock, C. H., 64

Hitterman, 225

Hobbs, W. H., 199

Hochstetter, F. von, 92

Holmes, W. H., 385

Hopkins, 275

Hovey, E. O., 215

Hunt, T. S., 81, 93, 137, 153, 191, 243

Hure, Comte de la, 166

Iddings, J. P., 36, 53, 67, 77

389

INDEX

Irving, E. D., 264
Johnstone, A., 167
Joly, J., 174

Jones, T. Eupert, 305, 306
Judd, J. W., 139, 270, 309
Julien, A. A., 174
Kahlenberg and Lincoln, 223
Kalkowski, E., 71
Kemp, J. F., 77, 81, 83
Kerr, W. C., 274
Kidder, J. H., 157
King, Clarence, 67
King, F. H., 370
Kingsley, 286
Klement, M. C., 139
Kuhn, M. L., 84
Layard, A. H., 281
Le Conte, J., 243
Lemberg, J., 17, 197
Leverett, F., 341
Lindgren, W., 221, 261
Livingstone, D., 161
Loewinson-Lessing, F., 122
Loftus, 281

Loughbridge, E. H., 354
Marbut and Perdue, 246
Marsh, G. P., 161, 387
McGee, W J, 289, 312, 386
Meister, 370

Merrill, G. P., 83, 138, 199, 221, 331
Mills, J. E., 153, 182, 261
Miiller, Eichard, 169
Munroe, C. E., 174
Muntz, A., 157, 181
Murakozy, K. V., 224
Murray, Sir John, 171
Neumayer, M., 290, 375
Newberry, J. S., 344
Nordenskiold, N. A. E., 228
Oldham, E. D., 300, 373
Orton, Edw., 109
Owen, D. D., 277
Packard, E. L., 103, 365
Palarsson, Abbe, 84
Penck, A., 265
Penrose, E. A. F., 215
Perkins, G. H., 61
Fetrie, J. F., 285
. Phillips, J. A., 77
Pirsson, L. V., 60
Potter, W. B., 252, 263
Proctor, J. E., 378
Pumpelly, E., 263, 272
Purrington, C. W., 265
Eath, G. vom, 241, 242
Eeade, T. M., 171

Eeclus, E., 335

Eeusch, H., 236

Eichthofen, F. von, 68, 80, 315

Eogers Bros., 168

Eose, G., 75, 84

Eosenbusch, H. von, 68, 70, 78, 83,

87, 88, 91, 92, 93
Eosler, H., 17
Both, G., 188

Eoth, J., 25, 68, 89, 96, 140, 225, 242
Eussell, I. C., 105, 180, 254, 265, 270,

284, 315, 320, 374
Eutley, F., 104, 172
Eutgers, J. W., 335
Safford, J. M., 254
Salisbury, E. D., 264, 275, 292, 340
Schutze, E., 210

Shaler, N. S., 160, 177, 291, 324, 379
Smith, Angus, 157
Smyth, C. H., 209, 236
Sorby, H. C., 35, 178, 329
Spencer, J. W., 271
Stanley, H. M., 161
Steinreide, F., 362
Stejneger, L., 128
Stone, G. H., 164
Streeruwitz, H. von, 161
Tafft, J. A., 291
Teall, J. J. H., 23, 70
Thompson, W., 233
Tornebohm, A. E., 84
Tschermak, G., 23
Udden, J. A., 280, 282
Upham, W., 339
Van Bemmelen, 362
Van Hise, C. E., 220, 253, 273, 354,

368

Von Buch, L., 79
Vom Eath, 78

Wadsworth, M. E., 64, 80, 240, 241
Walther, J., 245
Watson, Thos. H., 193, 203
Werner, A. G., 264, 316
Whitney, J. D., 264, 316
Whitney, M., 275, 296, 301, 329, 332,

367, 369

Wichman, A., 81, 149
Widogradsky, 181
Wiley, H. W., 155, 304, 385
Williams, G. H., 68, 81, 91, 93, 94, 95
Williams, J. F., 60, 197
Willis, Bailey, 48
Winchell, N. H., 285
Woodward, J. B., 164
Worth, H., 270
Zirkel, F., 35, 64

INDEX.

Adobe, 121, 320

chemical analyses of, 321

distribution of, 320

thickness of, 321
^Eolian deposits, 331

rocks, 138
Agalmatolite, 108
Alabaster, 109

Alaska, rock weathering in, 264, 270
Albite as a rock constituent, 16
Alkali of soils, nature of, 359
Alluvial cones, 50

deposits, 308

mechanical analyses, 31 i

plains, formation of, 277

plain of the Mississippi, 312
A Incite, analyses of, 210

weathering of, 209
Alumina of soils, 362, 363
Aluminum as a constituent of the

earth's crust, 5
Alum shales, 121
Amianthus, 107
Ammonia in atmosphere, 154
Amphibolite, 148

Amphibole, analyses of fresh and de-
composed, 19

Amphiboles as rock constituents, 18
Amygdaloid, 85
Anacostia river, filling of, 311
Analyses, calculation of results, 188

of fresh and decomposed rocks,
discussion of, 191

of granite, discussion of, 187
Anamesite^ 87
Andesites, the, 79
chemical composition, 79

classification of, 80

colors of, 80

mineral composition, 79

nomenclature, 80

structure, 79

weathering of, 207
Anhydrite, 110

Animals effective in promoting de-
composition, 182
Animal life, effect on soils, 328
Anorthite as a rock constituent, 16
Anthracite coal, 131

Ants as promoters of decomposition,
182

effect on soils, 379
Apatite, 111

as a rock constituent, 25
Aplit defined, 63
Apo-rhyolite defined, 68
Aqueous rocks, 99
Aragonite as a rock constituent, 24
Arenaceous rocks, 113
Argillaceous rocks, 117
Argillite, analyses of, 214

composition of, 119

weathering of, 213
Arkansas river, sediment in, 277
Atmosphere, action of, 154

composition of, 154
Augite altered to hornblende, 36

porphyries, see melaphyr, 85

vitrophyrite, 86
Augitite, 95

chemical composition of, 96

Bacteria, action of, 181

effect of, 385
Barite, 110
Barium as a constituent of the

earth's crust, 7
Basalt, analyses of, 205, 206

Bohemia, weathering of, 205

France, weathering of, 206
Basalts, the, 86

chemical composition, 86

classification, 86

colors, 86

mineral composition, 86

nomenclature, 86

structure, 86

Basanite, see Tephrite, 88
Bastite, 21

Bat guano, analysis of, 361
Beach sands, 329
Beauxite, 102
Bermuda, sand of, 329

limestone, weathering of, 233
Biotite as a rock constituent, 22
Bituminous coal, 130
Black earth of Russia, 316
Bog deposits, classification of, 306

391

392

INDEX

Bog deposits, depth of, 306
Bone breccia, 132

Boss-like form induced by weather-
ing, 231
Bosses, 46

Botryoidal structure, 34
Boulder clay, 340
Boulders, discoloration of, 244

of decomposition, 230
Breccia, 115

formed by weathering, 237
Bronzite, 21
Bronzitite, 95
Brown iron ore, 101
Cabook, origin of, 228
Calcareous group of rocks, 121, 124

rocks, weathering of, 216
Calcite as a rock constituent, 24
Calcium as a constituent of the

earth's crust, 6

Calcite, mode of weathering, 236
Calcium carbonate, amount annually

removed in solution, 171
Calc sinter, 104

tufa, 104
Carbon as a constituent of the

earth's crust, 7

Carbonaceous group of rocks, 129
Carbonates, production of in wea-
thering, 253
Carbonic acid, amount brought down

by rainfall, 157
in the atmosphere, 156
in water, influence of, 169
Ceylon, weathering of granite in,

228

Chalcedony, 103
Chalk defined, 125

cavities in, formed by weather-
ing, 247

Champlain clays, 118, 322
Chemical action of water, 165

composition of rocks, 41
Chert, 103

weathering of, 215
Chinese loess, 315
Chlorides, 111

Chlorite as a rock constituent, 27
Chrysotile, 107

Citric acid, solvent action of, 181
Clastic rocks, 112
Clays, aqueo-glacial, 288, 322
boulder, 340
Champlain, 322

mineral nature of 323
origin of, 323
chemical analyses of, 324
Clay, defined, 117

Clay, effect on soils, 368
-iron-stone, 106
slates, 119
Climate, influence on weathering,

263

Clinton iron ores, origin of, 254
Coastal plain deposit, 300
Coal, anthracite, composition and

origin, 131
bituminous, 130
Colluvial deposits, 307
Color changes due to weathering,

243, 244
of rocks, 42
of soils, 373

Concretions, formation of, 32
Concretionary structure, influence on

weathering, 232
Conductivity of rocks, 162
Conglomerate, 115
Contact metamorphism defined, 136
Coprolite nodules, 132
Coquina, 125

Corsica, granite weathering in, 236
Creeping of soil cap, 274
Cumulose deposits, 301

rate of formation, 305
Crystalline schists, the, 146
Daubree's experiment, 176
Decay, time limit of, 260
Decomposition of rocks, see weather-
ing, 150

during trituration, 176
Delta deposits, character of materi-
als, 309

mechanical analyses, 310
Deoxidation, 166
Desert varnish, 244
Deweylite, 108
Diabase, Chatham, Va., analyses of,

204

weathering of, 203
mandelstein, 85

Medford, Mass., chemical an-
alyses of, 200
porphyrite, 85
weathering of, 198
weathered, mechanical analyses

of, 199
Diabases, the, chemical composition,

82-83

classification, 84
colors, 84

mineral composition, 82
structure, 83
Diallogite, 95

Diamond Head, Oahu, sand of, 329
Diatomaceous earth, 123

INDEX

393

Dikes, 464

Diorite, analyses of, 207

-andesite group, 76

origin through metamorphism,
136

weathering of, 207
Diorites, 76

chemical composition of, 77

classification of, 77

colors of, 77

mineral composition of, 76

structure, 77

Distintegration without decomposi-
tion, 227
Ditroite, 75
Dolerite, 86

weathering of, 270
Dolomite, 126

as a rock constituent, 24

chemical composition of, 127

origin of, 138

weathering of, 236
Dolomites, crystalline, chemical and
mineral composition, 141
colors of, 141
nomenclature, 142
Drift, galcial, 277
Drumlin, 50, 314
Dunite, 92
Dust in snowfall, 332

soil, chemical analysis of, 333

mechanical analysis of, 333
Dunes, sand, 50, 334
Dynamic metamorphism defined, 136

Earth 's crust, thickness of, 2

Earthworms, effect on soil, 381

Eclogite, 148

Effusive rocks, characteristic struc-
ture, 57
defined, 56

Elaeolite. syenite, 73

weathering of, 196

Elements constituting rocks, 4

Enstatite, 21

Eozoon canadense, nature of, 138

Epidiorite, 84

Epidote as a rock constituent, 23

Erosion, material lost by, 175

Esker, see glacial deposits, 277

Eukrite, 84

Exfoliation of granite, 231

Expansion and contraction, effects

of, 158

of minerals, coefficient of, 255
of rocks by heat, amount of, 159

Fault, definition of, 49
Feldspars as rock constituents, 13

Feldspars, decomposition of, 17

importance of, 16

weathering of, 223
Felsite-pitchstone, 66
Felsophyr, 66
Felstone, 66
Fiorite, 103
Flint, 103

Flood plains of rivers, 277
Foliated or schistose rocks, 142
Forellenstein, 82
Forests, influence of, 266
Fossiliferous limestone, structure of,

127

Fourchite, 75

Foyaite-phonolite group, 73
Frontal moraine, 343

aorons, 344
Frost action on soils, 356

heaving effect of, 382

protective action of, 264

Gabbro-basalt group, 80
Gabbros, the, 80

chemical composition, 81
classification of, 82
colors, 82

mineral composition, 80
structure, 81
Geest, 289

Gem sands, origin of, 253
Geyserite, 103

Glaciated area, extent of, 339
Glacial deposits, 278, 338

classification of, 340
thickness of, 344
drift, section through, 345
ice, disintegrating action, 179
sheet, drifting power of,

279
sheets, effect on landscape,

279

lakes, filling of, 277
soil, composition of, 352
Glauconite as a rock constituent,

Glauconitic sand, 116-117
Glaucophane schist, 148
Gneisses, the, 142

the age and occurrence of,

143
classification and nomenclature,

143, 146
colors of, 143

mineral and chemical composi-
tion, 142, 145
origin of, 144

394

INDEX

Gneisses, structure of, 143 ,
Gneissoid granite, N. Garden, Va.,
chemical analyses of, 194
weathering of, 192
Granite, chemical composition of, 62
classification and nomenclature,

color of, 63

Corsican, weathering of, 236
Granite described, 61

District of Columbia, chemical

analyses of, 186
mechanical analyses of,

190

weathering of, 185
geological age, 64
Greenville, Ga., analyses of, 195

weathering of, 195
-Liparite group, 61
mineral composition of, 61
mode of occurrence, 64
structure of, 62
Stone Mt., exfoliation of, 231
weathering of, 228
Granitell defined, 63
Granophyr, 66
Grauwacke, 116
Greensand, 116
Greenstone, 76
Greywacke, 116
Greisen defined, 64
Ground moraine, mechanical anal-
yses of, 342

nature of material, 341
Gruss, 289
Guano, analysis of, 132, 361

origin of, 131

Gypsum, composition, origin and
occurrence, 109

Halleflinta, 146

Hardness of minerals, scale of, 12

Hardpan, defined, 373

Harzburgite, 92

Heat and cold, effect on rocks, 158

Hematite, 100

as a rock constituent, 26
Hornblende as a rock. constituent, 18

decomposition of, 19
Hornblendite, 95
Horseback, 50
Hyalomelan, 87
Hyalotrachyte, 73

Hydrargillite a product of weather-
ing, 270

in soils, 362
Hydration, 166

importance of, 166

Hydration, incidental to weathering,

238

Hydraulic limestone, 127
Hydrometamorphism defined, 140
Hyperite, 82
Hypersthene, 21
Hypersthenite, 95

Ice, mechanical action of, 177
Igneous rocks, 55

Induration of rocks on exposure, 240
Insects, effects on soils, 378
Intrusive rocks, 56
Iron as a constituent of the earth's
crust, 5

ores, 25

pyrite as a rock constituent, 26
weathering of, 165

salts, solubility of, 225
Itacolumite, 116

Jasper, 104

Jointing as influencing weathering,
230

Kalk-diabase, 86

Kames, 50

Kaolin, 108

chemical analyses of, 298
composition of, 118-119, 298
distinguished from kaolinite,

297

deposits, origin of, 254
mechanical analyses of, 297
origin of, 118

Kaolinization defined, 17

Kersantite, 78

Keratophyr, 72

Kinzigkite, 148

Kissimmee valley, swamp deposits,
304

Kugel-porphyry, 66

Laccoliths, 46

Labradorite as a rock constituent, 16

Lake Agassiz, 278

Lakes, filling of, 302, 314

transient nature of, 314
Lapilli, 122
Laterite, 121, 298

composition of, 299
Laurvikite, 75

Lava, cause of structural features-
57

defined, 46
Leda clays, 322
Leopardite, 66
Leucite as a rock constituent, 18

INDEX

395

Leucite-nepheline rocks, the, 96

chemical composition of, 96
classification of, 97
colors of, 96

mineral composition of, 96
nomenclature of, 97
structure of, 96
Leucitite, 97
Leucitophyr, 76
Leucophyr, 84
Lherzolite, 92
Lichens, action of, 180
Liebnerite, 75
Lignite, defined, 130
Limburgite, 93

composition of, 90
Lime and magnesia, relative solu-
bility, 225
carbonate, decomposing action

of, 355
in soils, 355

Limestone and dolomite, age of, 142
analyses of, 217, 219
chemical composition of, 127
color, variations of, 126
oolitic, 105
of Bermuda, 233
relative solubility of, 170
saccharoidal, 141
weathering of, 217
crystalline, chemical and mineral

composition of, 141
colors of, 141
nomenclature of, 141
structure of, 141
Limonite, 101

as a rock constituent, 26
Liparites, the, 66

Liparite, chemical composition of, 67
classification of, 68
color of, 67

mineral composition of, 66
nomenclature of, 68
structure of, 67
Litchfieldite, 75
Loess, 121, 315

characteristics of, 315
chemical analyses of, 318
distribution of, 316
mechanical analyses of, 319
microscopic examination of, 317
of Europe, 316
of the United States, 317
section of, 319
Luxullianite, 64
Lydian stone, 104

Magnesian limestone, 126
Magnesia, relative solubility of, 225
Magnesite, 106
Magnesium as a constituent of the

earth's crust, 6

Magnetite as a rock constituent, 25
Man, ravages of, 387

as a geological agent, 385
Manganese as a constituent of the

earth's crust, 7
oxide, 101

Marble, definition of, 126
Marls, composition and origin of,

128

Melaphyrs, the, 85
classification, 85
colors of, 85

mineral composition of, 85
nomenclature of, 85
structure of, 85
Melilite basalt, 87
Menaccanite as a rock constituent,

Metamorphic rocks, 135, 140
Metamorphism defined, 135
Metasomatosis, 137
Miascite, 75

Mica as a rock constituent, 21
Microcline as a rock constituent, 15
Microgranite, 66

Microscopic structure of rocks, 35
Microscope, utility of in study of

rocks, 36
Mineral matter dissolved in river

water, 171
nature of soils, 362
Minerals, coefficient of expansion of,

255

constituting rocks, 9
relative durability of, 235

resistance to weathering,

221

, soluble in carbonic acid, 169
Minette defined, 69
Mississippi flood plain, 312

river, sediment carried by, 276
Monazite sands, origin of, 254
Monzonite defined, 70
Moraineg, character of, 340

classified, 278
Morainal material, distributed by

water, 344

Mountains, height limited by weath-
ering, 265
Mucky soils, 314
Muscovite as a rock constituent, 21

Napoleonite, 77

396

INDEX

Nepheline as a rock constituent, 18
rocks, the, chemical composition
of, classification of, colors of,
mineral composition of, no-
menclature of, structure of,
97
(Elaeolite) syenites, 73

chemical composition of, 74
classification of, 75
colors of, 74

mineral composition of, 73
nomenclature of, 75
structure of, 74
weathering of, 196
Nephelinite, 98
Nevadite defined, 68
Nile, delta of, 309

valley, cause of fertility, 313
Nitrates in soils, 360, 361

origin of, 361

Nitric acid in the atmosphere, 155
Nitrogen, effects of on rocks, 154

of soils, 360
Norites, 82
Novaculite, 104
Nummulitic limestone, 125

Obsidian defined, 68

Oligoclase, as a rock constituent, 16

weathering of, 228
Olivine, alteration of, 23

as a rock constituent, 22
Oolitic limestone, 125
origin of, 105
structure of, 126
Opal, 103
Ophiolite, 108
Ophite, 84
Organic acids, effects of, 174

matter in soils, 314
Original constituents of rocks, 10
Orthoclase as a rock constituent, 14
porphyries, 71

chemical composition of, 71
classification of, 71
colors of, 71
nomenclature of, 71
structure of, 71
Orthophyr defined, 71
Osar, see glacial deposits, 278, 338
Ouachitite, 75
Overwash plains, 344
Oxidation, 165
Oxygen, as a constituent of the

earth's crust, 5
of the atmosphere, 158

Paludal deposits, 324
Pantellerite defined, 68

Peat, composition and origin, 129
Pebbles, normal form of, 331
Pegmatite defined, 63
Pelites, 117
Peperino, 122

Peridotite-Limburgite group, 89
Peridotites, the, 90

chemical composition of, 90

classification of, 90

colors of, 91

mineral composition of, 90

nomenclature of, 91

structure of, 91
Phonolites, the, 75

chemical composition of, 76

classification of, 76

colors of, 76

mineral composition of, 75

nomenclature of, 76

structure of, 76
Phonolite, Bohemia, analyses of, 198

weathering of, 197
Phosphates, 111
Phosphatic group of rocks, 131

sandstone, 111, 132
Phosphorite, 111
Phosphorus as a constituent of the

earth's crust, 7
Phyllite, 148

Physical and chemical properties of
of rocks, 30

manifestations of weathering,

227
Picrite, 92

porphyrites, 92
Pisolitic limestone, 125
Plagioclase feldspars, 15
Plant life, effect on rocks, 378
Plants and animals, action of, 180
Plutonic rocks defined, 56
Forphyrites, the, 78

chemical composition of, 78

classification of, 78

colors of, 78

mineral composition of, 78

structure of, 78
Porphyritic structure, cause of, 58
Porphyroid, 146
Potassium as a constituent of the

earth's crust, 6
Potomac clays, mechanical analyses

of, 301
Potstone, 95
Primary rocks, 47
Protogine defined, 63
Propyllite, 80
Proterobase, 84
Psammites, 113

INDEX

397

Psilomelane, 101

Puddingstone, 115

Pulaskite, 75

Pumice, origin of, 57 ^

Pyrite as a rock constituent, 26

decomposition of, 165
Pyrolusite, 101
Pyrophyllite, 109

Pyroxene altered to serpentine, 107
Pyroxenes as rock constituents, 20
Pyroxenites, the, 93

chemical composition of, 94

classification of, 94

colors of, 94

mineral composition of, 93

nomenclature of, 94

structure of, 94

Quartz, 104

as a rock constituent, 12
pebbles, solution of, 172
porphyry, classification of, 66
color of, 65
composition of, 65
nomenclature of, 66
structure of, 65
-free porphyries, 71
Quartzite boulders, weathering of,

237

chemical composition of, 149
origin of, 137

Quitman Mountains, weathering in,
161

Eainfall, amount reaching soil, 267

Eegolith, the, 287

Eegur, 372

Residuary deposits, 289

chemical composition of,

294
inherited characteristics,

291

mechanical analyses, 296
physical properties, 291
Rhodochrosite, 106
Rhombporphyry, 71
Rhyolite, defined, 68

origin of name, 68
River, channels formed by weather-
ing, 229

water, amount of mineral mat-
ter in solution in, 171
Rivers, transporting power of, 309
Rock-forming minerals, list of, 11
Rocking boulders, 238
Rock classification, 53
Rocks, chemical composition of, 41
color of, 42

Rocks, conductivity of, 162
definition of, 1
formed as sedimentary deposits,

112
through chemical agencies,

igneous agencies, 55
fracture of, 44
kinds of, 52
igneous, 55

how to be studied, 60
lustre of, 44
mode of occurrence, 45
oldest known, 45
original constituents of, 10
physical and chemical proper-
ties of, 30
relationship of plutonic and

effusive forms, 59
secondary constituents of, 10
specific gravity of, 40
structure of, 30
structural features dependent on

conditions of cooling, 56
temperatures at varying depths,

162

thin sections of, 35
Rock weathering, see under weather-
ing, 150
defined, 151

Roots, corrosive action of, 181
depth of penetration, 180

Salt, distintegration through crys-
tallization of, 177
water marsh deposits, 324
Sand dunes, 283, 334

lithological nature of, 334
rate of movement, 334
types of, 329
Sands, JEolian, 283, 334

beach, 329
Sandstones, 114

cementing material of, 114
composition of, 115
Sandstone, spheroidal weathering of,

233

weathering of, 213
Santa Rosa Island, sand of, 329
Saxonite, 92
Schists, the, 146

chemical composition of, 149
mineral composition of, 146, 147
origin of, 148
weathering of, 234
Seacoast swamp deposits, analyses

of, 327
formation of, 342

398

INDEX

Seacoast swamps, reclamation of,

327

Secondary constituents of rocks,
10

rocks defined, 47

formation of, 47
Sedentary materials, 288
Sedimentary deposits, 112

rocks, classification, 113

defined, 49

Sedimentation, process of, 48
Selenite, 109
Septarian nodule, 33, 106
Sericite as a rock constituent, 22
Serpentine as a rock constituent, 27

derived from olivine, 23
pyroxene, 107

origin of, 137

weathering of, 209
Shales, composition and origin, 120
Shale, word defined, 120
Shell limestone, 125

weathering of, 219

marl, 128

Sheets, intrusive, 46
Silica, 103

lost during weathering, 220
Siliceous sinter, 103

group of rocks, 123
Silicon as a constituent of the

earth's crust, 5
Sills, 46

Silt from granite, analyses of, 191
Singing sands, 125
Sinkholes, formation of, 247
Slates, 119

roofing, origin of, 135
Slickensides, defined, 49
Snow, dust in, 332
Soapstone, analyses of, 211, 212

composition of, 95

origin of, 95

weathering of, 211
Sodium as a constituent of the
earth's crust, 6

chloride, 111
Soil, the, 345

capacity for water, 368

chemical nature of, 345

creep, 274

definition of, 3

fertility, cause of, 349

inorganic nature of, 346

temperatures at varying depths,
162

water contents of, 267
Soils, affected by plants and animals,
378

Soils, alkali of, 359

action of frost on, 356

cause of color variation, 374

classification, 371

color of, 373

colloidal matter of, 365

compared with the mother rock,

348
essential constituents of, 350,

351
fertility dependent on physical

conditions, 368
due to lime, 355
grains in one gram, 367
interspaces of, 367 .
leaching of, 356
mineral nature of, 362
nitrates in, 360, 361
nitrogen of, 360
of arid regions, 357

and humid regions com-
pared, 356-359
of Seychellian island, 362
/physical condition of, 367
regur of India, 372
residual, analyses of, 347
ruined by erosion, 386
soluble matter in, 354, 365,

366

soluble salts in, 357
water capacity of, 368, 370
weight of, 371
wind drifted, 284
zeolites in, 363
Solution, amount lost in, 245

of rocks, 168

Specific gravity of rocks, 40
Spheroidal structure due to weather-
ing, 232

Sphagnous deposits, thickness of, 306
Sphagnum, rate of growth in bogs,

305

Spilite, 85
Stalactites, 106
Stalagmites, 106
Steatite, 108
Stone Mountain, Ga., weathering of,

231

implements, weathering of, 260
Stratified, or bedded rocks, 141
Structure of rocks, 30
Subsoil, defined, 373
Sulphur as a constituent of the

earth's crust, 8
Sulphuric acid, corrosive effects of,

172

Surface contours incidental to wea-
thering, 246

INDEX

399

Swamp deposits, analyses of, 304

mechanical analyses of, 328
seacoast, analyses of, 327

formation of, 342
reclamation of, 327
Syenite, Arkansas, analyses of, 196

weathering of, 196
Syenites, the, 69

chemical composition, 69, 70
classification of, 69
colors of, 69
mode of occurrence, 70
mineral composition, 69
nomenclature of, 69
-trachyte group, 68
structure of, 69

Table mountains due to weathering,

239

Tachylite, 87
Talc, 108
Talus, defined, 50
Temperature changes, effect of in

Arabia Petrea, 161
in Lower California, 161
in Massachusetts, 160
effect on rocks, 158
effect of, in Texas, 161
Tephrites and basanites, 88

chemical composition of, 89
classification of, 89
colors of, 89

mineral composition of, 88
nomenclature of, 89
structure of, 89
Terminal moraines, 343
Terra rossa, 375
Teschenite, 84

Theralite-basanite group, 88
Theralites, the, chemical composition
of, colors of, mineral composition
of, structure of, 88
Thin sections of rocks, how prepared,

Till, see ground moraine, 340
Toadstone, 66
Tonalite, 78
Trachytes, the, 72

chemical composition of, 72
classification of, 73
colors of, 73

mineral composition of, 72
nomenclature of, 73
structure of, 72

Transportation of rock debris, 274
by gravity, 274
water and ice, 275
wind, 280

Trap rocks, 85

Trass, 122

Travertine, 105

Trees, uprooting, effect of, 384

Trowlesworthite, 64

Tuffs, 122, 133

Valley drift, see glacial deposits,

344

formed through weathering, 240
Variolite, 84
Veins, defined, 49
Verdantique, 108
Vitrophyr, 66
Vogesite defined, 70
Volcanic ashes, 122
dust, 336

chemical composition of,

134

drifting of, 286
fragmental rocks, tuffs, 122
necks, 46

Wacke, 300

Wad, 101

Water, amount in soils, 370

and ice, mechanical action of,

175

transporting action, 275
capacity of soils, 368
chemical activity augmented,

183

effect on dry soil, 369
expansive force of, 177
relative solvent power of salt

and fresh, 173
protective action of, 239
relative run off, 368
solvent action of, 168
transporting power of, 276
Waves, action of, 176
Weathering, amount of material lost,

220, 273

as affected by forests, 266
changes in color caused by, 243,

244
difference in kind in warm and

cold climates, 269, 270
effacement of characteristics by,

249

final product of, 362
formation of cavities by, 242
forms assumed, 238
incidental discoloration, 242

surface contours, 246
influenced by humidity, 257
by composition, 256
by mineral composition, 234

400

INDEX

Weathering influenced by crystalline
structure, 229

by structure of rock

masses, 230
of alnoite, 209
of andesite, 207
of argillite, 213
of Arkansas syenite, 196
of basalt, 205
of chert, 215
of calcareous rocks, 216
of calcite, 236
of diabase, 198, 203
of diorite, 207
of dolerite, 270
of dolomite, 236
of gneissoid granite, N. Gorden,

Va., 192
granite in Ceylon, 228

in Corsica, 236

in District of Columbia, 185

in Greenville, Ga., 195

in Madagascar, 237
of greenstone, 229
of limestone, 219, 233
of phonolite, 197
of prehistoric implements, 260
of quartzite boulders, 237
of rocks, 150

defined, 151

early opinions regarding,
152

principles involved, 151

resume, 220
of sandstone, 213, 233
of schists, 234
of sedimentary rocks, 212
of serpentine, 209

Weathering of syenite, 196
of soapstone, 211
of stone implements, 260
of ultra-basic rocks, 208
physical manifestations of, 227
polish due to, 242
pre-paleozoic, 262
relative amount of material lost

in solution, 245
relative rapidity of, 258

in warm and cold

climates, 263
rate of, influenced by position^

257

influenced by texture, 255
results due to position, 238

incidental to, 253
simplification of compounds by,

252

Websterite, 95
Wehrlite, 92
Williamsite, 108
Wind action, 280
Wind-blown sand, abrading power,

163

Wind-drifted soil, 284

Wind, effects of, 163

erosion, 280

Zeolites, as conservators of potash,

365

as rock constituents, 28
composition of, 29
formation of, 365
possible occurrence in soils, 28,

363
Zircon syenite, 75

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